The 20th Century includes all events that took place from January 1 of year 1901 to December 31 of 2000.
- Date Unknown: Stefan Wasserman is born. At 13, he will lie about his age to enlist in the military and serve in World War I.
- Date Unknown: The events of "The Last Bounty Hunter" occur. Of note, Jonah Hex is shot and killed by George Barrow.
- January 17 Earth-Two: Sven and Celestine Nelson give birth to their son, Kent. Kent Nelson is destined to one day become the sorcerer Doctor Fate.
- April 15: The RMS Titanic sinks in the North Atlantic. This is cause by the Leviathan, built by Arthur Gordon Pym.
- Date Unknown: The last known passenger pigeon goes extinct in the town of Hatton Corners.
- July 28th: Austria-Hungary declares war on Serbia after it failed to meet the conditions of an ultimatum following the killing of Archduke Francis Ferdinand by a Serbian assassin. This event leads to the outbreak of war. Known as the First World War, the conflict continues until 1918.
- August: The Prohaska family moves from Krakow to the slums of Lviv. Janos Prohaska is two-years old at this time.
- May 30: Edmund Hathaway is killed when his destroyer is sunk off of Jutland. His father receives a telegram alerting him to his son's death a week later.
- June 6: Doctor John Hathaway of the Royal Museum in Scotland, steals the Magdalene Grimoire. He brings it to occultist Roderick Burgess in Wych Cross and begs him to use his skill to resurrect his dead son.
- June 10: Roderick Burgess and the Order of Ancient Mysteries conduct a ceremony aimed at capturing Death of the Endless. Instead, they manage to capture Death's sibling Dream, which causes "sleeping sickness" to numerous people including:
- Ellie Marston becomes a victim of the "sleeping sickness" and is later diagnosed with Enchephalitis Lethargica.
- In Kingston, Jamaica, Daniel Bustamonte is terrified to sleep and stays up until dawn.
- In Verdun, France, 13-year-old soldier, Stefan Wasserman is in a state of shell shock and refuses to go to sleep.
- Unity Kincaid suffers from the sleeping sickness and lapses into a near coma-like state.
- Date Unknown: Baron Winters flees Russia during the Russian Revolution.
- Date Unknown: Shell shocked Stefan Wasserman is discharged from the military. He is fifteen-years-old.
- July 7, 7:00 a.m.: Johnny Thunder is born "in the seventh circle of the moon Lahseen." According to Badhnisian prophecy, such a male child, on his seventh birthday, would have the power to rule the world and the Badhnisians wanted to rule the child.
- August 1: Etrigan participate in the Third Battle of Ypres, killing both British and German soldiers.
- Summer: Johnny Thunder (around 1 year old) is kidnapped by Badhnisians and brought to the oriental country of Badhnisia and the Temple of the Seven Great Gates where they conduct a mystical ceremony and adorn him with the Eternal Zone of the Zodiac - a belt that will give Johnny "world-dominating power" on his seventh birthday. But Badhnisia's neighbor, Agolea, fearful of that power, declares war on Badhnisia and wins. The woman who had originally kidnapped Johnny is charged with raising him in secret and takes him to a small fishing village north of Brunei, Borneo.
- June: Dr. John Hathaway of the Royal Museum commits suicide. His suicide note implicates Roderick Burgess in a scandal. An inquest is held, but Burgess is eventually acquitted of any wrong-doing.
- After July: Johnny Thunder is now five years old and living in Borneo. He is playing in a fishing boat when a stiff wind carries him out to sea. He is found by an American freight ship by crewman Congreve, who, upon reaching New York City, is charged with taking him to the police. As they board a streetcar, Johnny is recognized by his father, Simon B. Thunder, who now works on the streetcar. He takes Johnny home again.
- July 7: It rains for seven consecutive days all over the Earth. This sign from the heavens lets the High Priest of Aissor know that Johnny Thunder is still alive and that the ceremony to give him world-dominating power on his seventh birthday (i.e. today) was successful. He sends people to scour the globe looking for Johnny. They will not find him for sixteen years.
- August: Young Alexander Burgess finds a picture of Dream in the Paginarum Fulvarum. He shows it to his father and they now know the identity of the being they have held captive for more than 10 years.
- August: Ruthven Sykes begins having an affair with Ethel Cripps. Ethel Cripps is the former mistress of Roderick Burgess. Note: It is possible that the affair began much earlier than this, but a photograph dated at this time clearly shows that the two are involved with one another as early as 1926.
- Date Unknown: General Chiang Kai-shek of China sends Enemy Ace, Bat Lash, Biff Bradley, Chop-Chop and General Joseph Warren Stillwell to Dinosaur Island to recover the Swords of Fan. On this mission, the group encounters Vandal Savage and Miss Fear.
- Date Unknown, Earth-S: Billy Batson is born. When he is approximately ten-years-old he will become the super-hero known as Captain Marvel. Note: Date approximated based upon relative age of Batson at the time of his first appearance.
- November: A schism brings chaos to the Order of Ancient Mysteries. Member Ruthven Sykes betrays the Order and steals a large sum of money, as well as several mystical artifacts that once belonged to Dream. He and Ethel Cripps move to San Francisco.
- December: Ruthven Sykes sells the helm of Dream to the demon Choronzon for protection. In exchange, Choronzon enchants the Dreamstone, giving Sykes protection against the vengeful Roderick Burgess so long as he wears it.
- Date Unknown: Desire of the Endless rapes the sleeping Unity Kincaid, impregnating her. This sets into motion a chain of events that will culminate with the death of Desire's brother Dream.
- Date Unknown: Gangsters murder a bootlegger named Peter. Peter's lover, Alice Cohen, desires revenge and collaborates with her friends, Han and Rabbi Reva, to animate a stone golem with a mystical tome known as the Sefer Yetzirah. The immortal golem later becomes known as the Monolith.
- Date Unknown: Janos Prohaska flies with the Bill Heywood Squadron.
- Date Unknown: Ethel Cripps leaves her lover Ruthven Sykes and steals the Dreamstone from him. No longer protected by the amulet, Sykes is now vulnerable to attack. Occultist Roderick Burgess uses his magic powers to kill Sykes for betraying the Order of Ancient Mysteries in 1930.
- Date Unknown, Earth-Two: Auburn Automobile begins distribution of the Cord 810. Millionaire Bruce Wayne purchases one and uses it as his first "Batmobile" before customizing it.
- July: The Spanish Civil War begins. British actor and aviator Death Mayhew serves as a pilot.
- August: The Summer Olympics open in Berlin, Germany. Libby Lawrence competes in the women's swimming competition and William Everett competes in long-distance running.
- Date Uncertain: Polo player Rodrigo "Rodney" Gaynor happens to enter the town of Seguro, New Mexico, where he learns of an expected lynching of a local man and the planned inaction of the Sheriff. Escorting a Miss Dillon, who is passionate about saving the man, to the church, Rodney discovers the priest has the costume and whip of the legendary hero El Castigo. Rodney takes up the identity of "The Whip" and helps rescue the boy.
- July: Wesley Dodds no longer suffers from nightmares. With his new calling as a mystery man, he now sleeps the "Sleep of the Just". Note: This was likely intended to coincide with the first time that Wes Dodds becomes the Sandman. However, Sandman Mystery Theatre #1 retroactively establishes that Dodds began adventuring as the Sandman as early as 1938, not 1939.
- August 14: While exploring the Tomb of Kha-Ef-Re in Egypt, Daniel Garrett and Luri find the Blue Beetle Scarab. Shortly after collecting it, Dan uses the power of the Blue Beetle to save Luri from a cave-in.
- September 1: Nazi Germany invades Poland, beginning the Second World War.
- December 24 through 31: Rex Tyler uses his Miraclo formula for the first time, becomes Hourman, and has his first solo adventure.
- February: Police Detective Jim Corrigan is killed by Gat Benson, but is charged with an eternity of fighting evil on Earth as the Spectre.
- On or after July 7: Johnny Thunder, now 23, accidentally summons the Thunderbolt for the first time and saves the life of a window washer. Soon afterward, he accidentally summons the Thunderbolt again while being ambushed by Badhnisians seeking to return him to their home country so his powers can be used by the High Priest. In neither event did Johnny learn how he was able to summon the Thunderbolt and, as the Thunderbolt is unable to communicate it, Johnny must figure it out on his own.
- November 14: O.S.S. agent Paul Woods is released from a London hospital with a metallic scalp-plate. He takes part in a suicide mission to destroy a Nazi broadcasting tower which aids the German military in successful bombing raids in England by using his scalp as an antenna, allowing a bomber plane to target his location once he entered the tower.
- December: The formation of the first incarnation of the Justice Society of America - first telling.
- April: Mexican bandit José Salvez kidnaps Joan Williams and takes her to Mexico. The Flash races across the border to rescue her.
- April: Green Lantern investigates incidents of sabotage at a steel mill. The saboteur kidnaps Irene Miller, and Green Lantern has to rescue her, while also exposing the criminal's affairs.
- April: Johnny Thunder gets a job working for millionaire Manny Bucks. One of Manny's stooges, Oscar Ovaseer, tries to get Johnny fired, but Johnny exposes Oscar's illegal activities.
- April: Johnny Thunder saves Daisy Darling from a fake fortune teller named Mogli the Magnificent.
- April: A corrupt police officer named Corrigan (not to be confused with Jim Corrigan), frames the King for the murder of Silas Hackett. The King disguises himself as Hackett's ghost in order to expose Corrigan's involvement.
- December 6: Per Degaton travels backwards in time from the year 1947 with several super-villains in a bid to take control of the world. His cronies succeed in capturing the most powerful members of the Justice Society of America.
- December 7: The Japanese Navy stages a surprise attack against the U.S. Naval base at Pearl Harbor. Naval officers Rod "Firebrand" Reilly and Slugger Dunn are believed killed during the attack. In response to the sneak attack at Pearl Harbor, President Roosevelt orders those available Justice Society of America members to lead a coalition of every masked mystery-man in the country under the banner of the All-Star Squadron. A team of heroes go to Hawaii where they manage to free the captured JSA and defeat Per Degaton and his henchmen. In the process, Danette Reilly gains fire-related superpowers and ultimately becomes Firebrand II.
- Date Unknown: In Egypt, the Gravedigger intercepts a German Tiger division in order to procure top-secret documents from a Nazi officer named Eric Vonker.
- May: Baron Blitzkrieg and his comrades Zyklon and Major Zwerg attack Independence Hall in an effort to steal the Liberty Bell. They fight with Johnny Quick and Liberty Belle, but succeed in escaping with the bell.
- July 15: The Fiddler (Isaac Bowin) kidnaps Joan Williams upon breaking out of prison suspecting that it will lure Flash (Jay Garrick) into his trap, which it does. Flash is temporarily captured and placed in a trap but escapes with the help of an anonymous man (implied to be his future self) who tells him it is "very important" that Flash remembers today's date. (The mystery is solved in Wonder Woman (Volume 2) #129.)
- November: Bugsy Siegel kills Stuff, sidekick to Vigilante (Greg Sanders). Vigilante disrupts Bugsy's business activities in response. Bugsy contracts the Dummy to kill Vigilante. The Dummy rigs a houseboat with explosives and activates it while Vigilante is within, causing all to believe he has died.
- Spring: The JSA educate a man named Dick Amber on the history of German warfare, dating all the way to the time of the Teutonic knights.
- April: Hawkman faces off against the Monocle for the first time.
- April 15: Thirteen members of the JSA act as honor guard for FDR's funeral in Hyde Park. The heroes vow to continue the war effort until the Axis powers are defeated. NOTES: According to Last Days of the JSA, this was the last time in the 40s that all of the JSA's members (to that date) were together, including Hourman, Mr. Terrific and the Spectre, who had already left the team. In an alternate timeline, the Spectre appears after the burial and sends the JSA on a final raid in Berlin, where they all perish in a failed attempt to stop Hitler from destroying the world with the Spear of Destiny. That timeline was negated by the JSA's entry into the Ragnarok cycle.
- April (late): The Unknown Soldier is sent on a mission to stop a Nazi super-weapon - vampiric octopuses called "Nosferatu".
- April 29: The Unknown Soldier infiltrates Hitler's bunker, killing him and assuming the dictator's identity in order to call off a weapons deployment. He then makes Hitler's death look like a suicide.
- May 1: The essence of a time-traveling Swamp Thing takes root in the body of U.S. soldier Alexander Holland. Holland assists Easy Co. in destroying a Nazi slaughter house/magus bunker run by Anton Arcane. Afterwards, Swamp Thing's spirit disappears again into the timestream.
- August 5: The Enola Gay drops an atomic bomb on Hiroshima, Japan, killing millions. One of the casualties is Thomas Morita, aka, Kung. Kung's spirit will eventually bring the 21st century JSA back in time to this event.
- June 12: Benjamin "Bugsy" Seigel asks his partners for more money to make sure the Flamingo Hotel is open December 26. They are not happy about the request. The Vigilante bides his time and recovers his health and nerves.
- December 26: Benjamin "Bugsy" Seigel's casino, the Flamingo Hotel, opens and is a bust. Dozens of confirmed celebrities fail to show. Bugsy's partners decide he must be killed as punishment. Meanwhile, the Vigilante returns to action, saving the lives of one of Bugsy's henchmen's wife and her adulterous lover.
- Date Unknown: Occultist Roderick Burgess pleads with the captured Dream for power. Dream refuses. Old and embittered, Burgess dies in front of him. Roderick's cult, the Order of Ancient Mysteries, passes on to his son Alex.
- January (date conjecture): Vigilante learns that Bugsy Siegel is targeted for asassination by the Dummy and intervenes (during which the Dummy is killed). Bugsy arranges for Vigilante to come to his home where he asks Vigilante to kill him (Bugsy doesn't want to spend the rest of his life looking over his shoulder and wants his killer to be a fearless man). When Vigilante explains his vendetta against him was about Bugsy's murder of Stuff, Bugsy doesn't even remember him. Vigilante decides to leave but Bugsy taunts him until Vigilante reconsiders and shoots him.
- January: Janos Prohaska, the Blackhawk, sits in a bathtub in a hotel in Singapore mewling over some disparaging comic strips. A man named Zelecki bursts into the room holding a knife. He demands the $10,000 that Prohaska owes him. Janos whips out a gun he had secured in the tub with him and shoots Zelecki in the hand.
- Date Unknown: Abe Crane joins the Central Intelligence Agency. He will one day become a Rook for Checkmate.
- Date Unknown: A spacecraft crashes in Kazakhstan. Word reaches the Soviet government and a team of Russian scientists come to explore the craft. Within, they find the male pilot dead from a chest wound, and a pregnant female passenger close to giving birth. They take the female back to their camp and begin working to deliver the baby. However, the female is superhumanly resistant to their medical equipment, and dies bringing the baby into the world. This baby is held in stasis by the Soviets for decades and referred to only as "Subjekt-17".
- Date Unknown: Booster Gold travels back in time to 1952 where he assists Task Force X in protecting Russian astrophysicist Sergei Pushkin. He also corrects the timeline by delaying manned space travel by several years.
- Date Unknown: Ellie Marston is diagnosed with Encephalitis Lethargica. She only wakes up four or five times a year.
- Date Unknown: Khalis the mummy awakens and battles Doctor Fate in Boston. Note: Takes place 15 years after Doctor Fate's debut in 1940.
- Date Unknown: The Order of Ancient Mysteries is but a shadow of its former self. Alex Burgess now uses it as a scheme to make money. His partner Paul McGuire is concerned that they still keep Dream imprisoned in the cellar.
- Summer, Earth-Two: Bruce Wayne and the reformed Catwoman, Selina Kyle are married.
- September 7; Earth-Two: Bruce and Selina Wayne give birth to their daughter, Helena. Helena Wayne will one day become known as the Huntress.
- Date Unknown: Doctor Emery Zackro passes away. Note: Date of death indicated on tombstone.
- August: Jimmy Olsen finds a magic totem that grants him three wishes. He creates a "Super-Girl" to help out Superman and briefly sends him back in time to meet his parents. Thieves steal the totem and use a wish to temporarily take away Superman's powers.
- Date Unknown: Salem, Massachusetts - Thomas Larson attempts to give Elizabeth a posey ring but instead she belittles him, telling him that the girls think he's daft and scary. All the while Thomas sees her aura as a solid red. Shortly after he dismembers her, crying and begging forgiveness.
- August: Criminal scientist Kurt Vornak sabotages Simon Stagg's laboratory in the hopes of destroying Metamorpho's chances for a cure. The equipment explodes, transforming Vornak into an Atomic menace.
- October: Metamorpho fights Nicholas Balkin who has been using the Telstar satellite system to cause electronic disasters across the globe.
- December: T.T. Trumbell creates Staggium, a metal that can defeat Metamorpho. He sends a Staggium robot to fight Metamorpho while Turnbull targets American missile sites. Metamorpho destroys the robot, and Turnbull is defeated when one of his own henchmen betrays him.
- Middle of the year: Danny Maddox, a Vietnam army man, is caught by USAF pilot Travis Morgan selling supplies on the black market. Danny arranges for Travis to be sent into Russia on a mission where Danny pilots a Soviet plane and attacks him. Both men are forced to eject. Travis discovers Skartaris and Danny is captured by Soviets and imprisoned in Siberia.
- September 14: Hourman (Rex Tyler) writes a letter to his son, Hourman (Rick Tyler), in which he recounts his origin story. The letter is ultimately delivered in Infinity Inc. #40.
- Appearances of 20th Century
- Location Gallery: 20th Century
- Note: Sandman (Volume 2) #1 indicates that Stefan was almost 14 in June of 1916, suggesting that he may have been born in either April or May of this year.
- DC Special Series #16
- All-Star Squadron #47.Note: In New Earth continuity, Kent Nelson was born in 1928 and instantly aged to adulthood.
- Young All-Stars #16
- The Brave and the Bold #54
- Historical record; supplemented by the Zero Hour timeline
- Secret Origins (Volume 2) #45
- Sandman (Volume 2) #1
- Night Force #1
- Secret Origins (Volume 2) #13
- The Demon (Volume 3) #0
- Guns of the Dragon #1-4
- Whiz Comics #2
- Monolith #1
- Blackhawk (Volume 2) #1
- 
- All-Star Squadron #23
- Booster Gold (Volume 2) #2
- Secret Origins (Volume 2) #16
- Secret Origins (Volume 2) #15
- G.I. Combat #240
- All-Star Comics #4
- DC Special #29
- All-Star Squadron #67
- Secret Origins (Volume 2) #31
- Flash Comics #16
- All-American Comics #25
- Justice League of America #193
- All-Star Squadron #1 through All-Star Squadron #3
- Men of War #15
- All-Star Squadron #45
- Speed Force #1
- Vigilante: City Lights, Prairie Justice #1
- Vigilante: City Lights, Prairie Justice #2
- All-Star Comics #24
- Flash Comics #64
- Last Days of the Justice Society Special
- Unknown Soldier #268
- Swamp Thing (Volume 2) #82
- Justice Society of America (Volume 3) #27
- Vigilante: City Lights, Prairie Justice #3
- Vigilante: City Lights, Prairie Justice #4
- Action Comics #601
- Checkmate #9
- Superman #656
- Booster Gold (Volume 2) #20
- 1st Issue Special #9
- DC Super-Stars #17
- DC Super-Stars #17 & Infinity Inc. #7
- Secret Society of Super-Villains #2
- Superman #123
- Nightwing (Volume 2) #129
- Metamorpho #1
- Metamorpho #2
- Metamorpho #3