Born in AC 180, Duo Maxwell was a then-nameless orphan in the L2 colony cluster, whose parents were assumedly killed in the war for independence with the Earth Sphere Alliance when he was very young. Growing up as a street rat with a group of other children who stole to survive, he became close to another street boy named Solo, who was older than him. When Solo became sick with a virus that swept through the colony, the children didn't have the money for the vaccine- Duo, who was one of the best thieves in the group, tried to sneak into the Alliance military base in order to steal the medicine, and succeeded. However, by the time he came back, Solo was already too far gone, and died in the company of Duo. Duo swore to his friend that he'd never forget him, deciding that 'if you're Solo, we'll be Duo'- so naming himself.
Not long after this, he and the group of streetchildren were caught while stealing food, and handed over to the local Catholic Church's head priest, Father Maxwell- who had run into Duo (quite literally) just before and volunteered to take in the children along with the nun Sister Helen. It was at this time that Duo began to sport his trademark braid, refusing to allow his hair to be cut- he was attached to it, as it was long because he was alive long enough for it to grow out that way. Duo surprised the people at the church with his worldliness, especially with the subjects of war and death- Duo believed there was another God, a God of Death- the Shinigami- because, as he said, 'he'd never seen any miracles, but sure seen a lot of dead people!'.
Duo came to live rather contently at the church for about a year. However this did not last as war broke out once again between the Alliance military and the colonist rebels, who forced the church to house their operations. Duo, seeing the colonists fighting endlessly, offered to steal a mobile suit for them to fight, as long as they left the church alone. Going to do so, Duo actually succeeded in stealing a Leo Mobile Suit- however, by the time he had returned, the church had been mercilessly wiped out, massacred by the Alliance military. Duo arrived back in time for Sister Helen to die in his arms, telling him how proud she and Father Maxwell had been of him.
Duo was arrested by the military, and the incident became known as the Maxwell Church Massacre- with Duo as the only survivor. He took on the surname Maxwell at this time in honor of his late adoptive father. Escaping at an unknown time before AC 192 Duo smuggled himself aboard a rebel ship belonging to the 'Sweeper Group'. Introducing himself as 'Duo Maxwell, who will run and hide but never tell a lie', the leader of the group, Professor G, asked Duo if he was Maxwell's demon, and then offered Duo the position to pilot the Gundam Deathscythe for a mission known as 'Operation Meteor.' Duo accepted.
Learning of what Operation Meteor truly entailed- massacring most of the Earth's population- Duo tried to detonate Deathscythe, refusing to allow his 'buddy' to be used as a tool for murdering innocent people. Professor G caught and stopped him, and told him to forget his orders for Operation Meteor, to 'steal' the Deathscythe and go to Earth to execute his new orders- take down the group known as OZ- as the God of Death (Shinigami).
Duo was sent down to Earth on April 7th, AC (After Colony) 195 as all the other Gundam pilots were, and immediately attacked an OZ base. Meeting Heero Yuy on one of his missions (shooting him at first, thinking him an enemy) and realizing that they were after the same goal, the two worked together on a few occasions, Duo breaking Heero out of a military hospital and salvaging their Gundams, before a supposed meeting of OZ leaders drew the two to meet the other 3 Gundam pilots. After that mission was failed and the accidental assassination of peaceful Alliance military leaders, Duo, along with Quatre and Heero, attempt to stop the detonation of the base they were on as OZ began its coup de etat over the Alliance.
Tricked into a trap by Colonel Une of OZ, the five Gundam pilots are almost forced to surrender, or the colonies will be destroyed. Rather than do this, Heero Yuy self detonates his Gundam- while he is still in it- and the other four pilots flee, now on the run. Duo flees with Quatre to an underground base run by an ally Middle Eastern country, whose government are supporters of Quatre's family, the famous Winner family. When OZ discovers them, they escape with the help of Quatre's backup soldiers, the Maguanac Corps.
Duo and Quatre, seeing the colonies led astray by a newly-made over, peaceful Une representing OZ, decide to go back to space to fight OZ's peaceful and deceiving takeover of the colonies. However, they are now deemed enemies by the colonies, and Duo's Gundam is captured by the new, more advanced mobile dolls that OZ has created, and Duo is arrested. Heero goes to kill him so no information is leaked to OZ, but instead decides to set him free- Duo lays low for a while before witnessing the Gundam Deathscythe being destroyed by new OZ recruits. This convinces him to keep on going, even though his 'friend' has been destroyed. He sneaks into an OZ recruitment center, stealing a transport to the moon base where mobile suits are being manufactured. It is then that he meets Hilde Schbeiker, an OZ recruit, and the talk they have makes her rethink her loyalty to OZ. Duo succeeds in getting to the moon, but is taken prisoner quickly afterwards- but not before discovering the 5 Gundam creators- including Professor G- at work on a new Gundam for him. In order to keep their development secret, he allows himself to be taken in.
As Heero and Wufei are also captured, and with Trowa undercover as an OZ recruit and Quatre missing, the boys hear of a new Gundam that has been destroying colonies. While Heero and Trowa go to fight it with OZ, Duo and Wufei are almost killed when their air supply is cut off by one of OZ's parent foundation's (Romefeller's) officials. Lady Une, learning of this, is shot in the process of restoring their air and setting them free, and Duo and Wufei escape, taking their new, but still incomplete Gundams with them.
Later, Duo has been salvaging stolen OZ parts to complete Deathscythe Hell, but is captured by a crazed OZ officer and forced to test the Wing Gundam ZERO- the ZERO system nearly drives him insane, as it does with all the other test subjects, but he escapes the cockpit without killing himself or anyone else, escaping in Deathscythe Hell as the OZ officer kills himself trying to use ZERO.
When the circus visits the colony Duo is hiding out on, he discovers Trowa has rejoined them- only to also discover that he has amnesia. Driven away by Trowa's friend and sister-figure Catherine, who hates soldiers and war, Duo goes to inform Quatre- the pilot who is closest to Trowa. Duo leaves the colony he has been staying on- Hilde's- for the safety of the colony, as he is still a wanted man. He instead regroups with Trowa, Quatre and Noin, and goes with them to the Peacemillion, a ship run by his friend Howard from the Sweeper Group. Attempting to stop the colonist resistance group White Fang, led by Zechs Merquise, all the Gundam pilots are reunited. When Hilde is almost killed attempting to steal enemy blueprints, Duo comes to her rescue, and reveals to Heero via Hilde that Relena is being held prisoner aboard the White Fang's ship Libra. Duo is reunited with the Gundam creators, who he assists in attempting to destroy the enemy battleship to end the decisive battle, though the creators are killed in the process. The battle is ended between the other Gundam pilots, and peace comes to the Earth and colonies at last on Christmas Eve, AC 195.
One year later it is the anniversary of the day peace was obtained- Christmas Eve, AC 196, and a new threat is sensed- someone plans to go through with the original Operation Meteor, which would mean dropping a colony onto the Earth. Helping Heero infiltrate Mariemaia Barton/Khushrenada's base on colony X18999, Duo allows himself to be caught in order to allow Heero and Trowa to continue the mission. Later breaking out and regrouping with Trowa and Sally Po, he regroups with all the pilots but Heero, and join the Preventer's fight on Earth against Mariemaia's invading mobile suit army.
After the war has ended, Duo returns to the L2 colony cluster and opens up a scrapyard and delivery business of his own, with the help of Hilde.