John Walker was born in Custer's Grove, Georgia to Caleb and Emily Walker. Instilled at an early age with patriotic feelings for his country he eventually joined the army like his brother Mike, who died in the Vietnam War. As soon as he was recruited the war ended and John's hopes of becoming an American Hero were crushed. He then left the military service were he heard of a person known as the Power Broker. When he learned from his friend that the Power Broker could give anyone superhuman powers, he sought out the Power Broker and signed up for the treatment with his friends. After he gained superhuman strength Walker was confronted by a man named Ethan Thurm, who convinced Walker to become the Super-Patriot. As Super-Patriot, Walker toured the country speaking at demonstrations and rallies. One day while in New York City Walker criticized Captain America's morals and ideals, saying that they were old and outdated. During his speech a group of men calling themselves the Bold Urban Commandos or "Buckies" attacked Walker. Walker easily defeated the Buckies and it is later learned that the Buckies were actually working for Walker at the time. Captain America eventually came into conflict with Walker and lost the fight.
Sometime later Walker was approached by Valerie Cooper and asked to become the new Captain America. Walker agreed instantly and asked his if former partners the Buckies could join him. The Commission on Superhuman Activities refused all but Lemar Hoskins who became the new Bucky. Together Lemar and Walker trained with Freedom Force's Blob, Pyro, and Avalanche. After a little time the Commission hired Taskmaster to train Walker to be able to fight like Captain America. Meanwhile Steve Rogers used the identity of the Captain and tried to reclaim his title of Captain America. During his time as Captain America the terrorist group known as the Watchdogs killed his father and mother. Also during his time as Captain America two of his former friends became Left-Winger and Right-Winger, whom he killed while in a rage. Eventually Rogers and Walker encountered one another and battled until both Walker and Rogers learned that it was the Red Skull who was manipulating the two of them. After the battle Walker finally realized what the costume symbolized and returned it to Steve Rogers.
Months later, the Commission held a demonstration of their new operative, U.S.Agent. In the demonstration U.S.Agent faced off against the Iron Monger and easily defeated the armor and its pilot. He then was sent on several solo missions including one against a British team called the Super Soldiers. On one of his early solo missions he joined forces with Captain America to stop a faction of Watchdogs. During this battle Captain America was worried about what Walker might do to the Watchdogs since they killed his family. However Walker restrained himself and proved himself worthy to Captain America.
Later, he was ordered to join the Avengers West Coast by the government in order to keep tabs on them. While in the Avengers he had frequent fights with fellow Avenger Hawkeye and fought many villains such as Magneto, Doctor Demonicus, Ultron, Hydro-Man, Grim Reaper, and several others. During one mission the Avengers West Coast fought the Bogatyri, a Russian mercenary group who had allied themselves with Doctor Demonicus and Klaw. Throughout these battles between the two teams U.S.Agent formed a grudge with Zevzda Dennista. After a battle on Doctor Demonicus's island a reactor exploded and the island began to sink. Overcoming their grudge Walker asked Zevzda if she wanted a ride home. She declined and instead kissed U.S.Agent.
While still an active Avenger, Walker continued to work on solo missions which included trying to bring down the Scourge of the Underworld organization, including its creator Thomas Halloway, but due to lack of evidence the courts could not put Thomas in jail. One of his solo missions included stopping a riot and a superhuman powerhouse named Peacemonger from terrorizing a local neighborhood. Another mission forced him to confront Nomad, who at the time had a warrant out for his arrest. Another one of Walkers most noted solo escapades was when he sought out a minority killer in Texas who kept killing all the Hispanics who came into town.
When the Avengers West Coast was disbanded, U.S.Agent was asked to join Force Works by Iron Man. He accepted the invitation and fought against many foes including the Mandarin and other foes. After Force Works disbanded U.S.Agent went solo again and encountered Wolverine and was critically injured by a person known as Protocide. After he recovered he led a team of government operatives called the Jury in order to apprehend the team called the Thunderbolts. Failing the Jury moved on and Walker was approached by S.T.A.R.S. who offered him a job. John accepted and worked for S.T.A.R.S. for a long time. During which he fought Captain America, stopped the Power Brokers vile plans, stopped an intergalactic plot by Ronan the Accuser, even managing to knock Ronan out.
Eventually, a government official named Dell Rusk, who little to Walkers knowledge was actually the Red Skull ordered the formation of a new team of Invaders. One of Walkers first priorities was to free Blazing Skull from Middle Eastern terrorists. Walker freed Todd and along with his fellow Invaders; Thin Man, Tara, Jim Hammond, Union Jack, Spitfire, Namor, the Fin, and many others freed the oppressed nation of Mazikhandar, eventually coming into conflict with the Avengers. During the confrontation he and Captain America clashed once again. After a long battle the enraged U.S.Agent was beaten by the Captain and tied up. In order to calm him down Hawkeye shot Walker in the butt, but contrary to his belief this only made Walker madder. After the Thin Man killed the ruler of Mazikhandar, it became a protectorate of Atlantis.
The Invaders continued to work on international missions, leading them to come into conflict with Axis Mundi members Baroness Blood, Meranno, Warrior Woman, Agent Axis, Gotteskrieger, and the cloned army of insectoid terrorists known as Pterorists. On one of his many missions with the Invaders, Baron Strucker sent Wolverine who was under the control of Hydra, to kill the members of the Invaders, specifically Namor. During the battle that followed Blazing Skull's left arm and leg was sliced off. Walker ordered the Skull to heal himself as he went to face Wolverine on his own. As Wolverine was about to kill Namor, Walker stabbed Wolverine in the back with his shield. Walker then smashed Wolverine to the ground and beat him to a pulp. He then continued to beat up Wolverine by tossing him near an explosive canister. U.S.Agent then shot at the canister causing it and Logan to burst into flames. Once Wolverine was thoroughly beaten Walker helped Namor get medical treatment. Afterwards Namor expressed his undying gratitude and the honor he had working with U.S.Agent. Later U.S.Agent attended the funeral of Jim Hammond and expressed his gratitude to the fallen hero.
After the Invaders seemingly disbanded Walker returned to the United States and donned his old Black costume and continued to work for the government. Recently he Civil War|registered with the Superhuman Registration Act and continued to serve his country. However the government and Tony Stark had different plans for him. Walker then snuck into Stark Tower and met with Tony when he asked to see him. In the meeting Tony told him that he was ordered to goto Canada and become a member of the new Alpha Flight Program. Enraged that they would think of sending him to Canada, Walker told Tony he was an American hero. Tony asked him once again, but Walker refused and left.
Sometime later Walker and a team of S.H.I.E.L.D. agents were assigned on a mission when the agents started to fire on Walker and a nearby helicopter. Walker started to flee thinking Tony was trying to have him arrested. However when he noticed the helicopters crew was in danger he saved them. He then turned his attention to the agents and easily took them out. He then realized that it was the Purple Man who was controlling the agents, but before he could react the Purple Man took control of Walker. Purple Man ordered Walker to leave his shield and jump off of the flying vehicle they were in. Unable to refuse, Walker jumped and was severly injured in the fall. Later as Walker recovered, Tony approached him again and told him that S.H.I.E.L.D. information said that Purple Man was heading to Canada. Mad with the circumstances Walker agreed to be a member of the new Omega Flight Program and left for Canada.
Arriving in Canada John was reuinited with former West Coast Avenger, Arachne and apprehended the superhuman known only as Daisy. From there they joined the Omega Flight team to discuss Michael Pointer's membership on the team. Meanwhile the Wrecking Crew were terrorizing downtown Toronto and beating up fellow Flight member Sasquatch who in turn was possessed by the demonic Tanaraq. Also the Wrecking Crew were enlisted in the aide by a mystical entity and demonic hordes were freed from an undisclosed location to rampage Canada. Alerted to the information the team went into action and fought against the Wrecking Crew, the possessed Sasquatch, and the demon hordes. After both the Crew and Tanaraq were taken care of fellow Flight member Beta Ray Bill led the demons into a dimensional rift, which was destroyed by Pointer, trapping Bill with the demons. After the battle U.S.Agent stayed with Omega Flight and took revenge upon the Purple Man, reclaiming his shield.
After some time, John was contacted by Loki disguised as the Scarlet Witch in order to incorporate him into Hank Pym's latest version of Avengers. Teleported away from his Omega Flight teammates John helped this new incarnation of Avengers defeat the elder god Chthon and cemented his status and position on the new Avengers team.
After this new team of Avengers was recognized by G.R.A.M.P.A., a global peacekeeping agency, they faced several threats such as an incursion into the Baxter Building which lead to a confrontation with the Fantastic Four, fighting against the former Inhuman king known as the Unspoken, and coming into conflict with Norman Osborn's team of Avengers and a cosmic cube empowered Absorbing Man. After the Absorbing Man fiasco Norman Osborn pulled rank on U.S.Agent and as the Head of National Security he stripped Walker of his title as U.S.Agent and ordered him to leave his team of Avengers.
Unhappy with the situation, Walker rejoined his team of Avengers during the siege of Asgard to intercept the Thunderbolts from stealing the Spear of Odin. During the ensuing conflict Walker was targeted by Thunderbolts leader Scourge who, after a lengthy and epic struggle, proceeded to sever Walker's left hand and leg. After the Spear was recovered thanks to Paladin, Stature carried Walker away from the battlefield to get him medical treatment.
Recently, Walker was fitted with a regular prosthetic for his left arm and is confined to a wheelchair after the incident in Asgard. Walker was then put in the position of the warden of the Raft.