John Walker grew up idolizing his older brother Michael, who had been killed during the Vietnam War. He went into military service but felt he wasn't a 'hero' like his brother was. Walker underwent the Power Broker process to compete in the Universal Class Wrestling Federation. However, his agent convinced him that he should become a hero.
He became the Super-Patriot, arguing that he stood for America's 'true ideals' and began to publicly discredit Captain America. Three 'Bold Urban Commandos', fellow recruits from the Federation, assisted him by staging attacks and intimidating others.
Left-Winger and Right-Winger publicly revealed Walker's name leading up to the Watchdogs killing his parents. Walker retaliated by killing most of the Watchdogs. He captured Left-Winger and Right-Winger and used his old 'torchsword' to ignite explosives they were strapped to, leaving them horribly burnt and in comas. 
Eventually there was a confrontation with Captain America who was then calling himself the Captain. Walker was stopped by Rogers. The two confronted the Red Skull, who had been manipulating the Commission. In light of these events, Walker was deemed unworthy of the Captain America title due to his actions.
Walker's death was soon faked by a member of the Commission and he re-emerged as US Agent. He wore a variation of the costume that Rogers had worn as the Captain and using his vibranium disc as a shield. Walker continued to work for the Commission. He was first seen as the U.S. Agent, battling an Iron Monger as a test for the Commission. 
He was placed as a watchdog of Avengers West Coast.and the Vision by the Commission, as a condition to possibly get their government clearance reinstated.
He later rescued his former partner Battlestar from the Power Broker, and reconciled with him; John learned that his memories had been altered and that his parents were dead.
Working for his old Commission ties has led him into conflict with other heroes. Walker seems to have finally cut ties with them after one mission put him into conflict with War Machine and Hawkeye.
Walker joined the new Invaders squad acting as its Captain America; he in fact demanded to be called by that name. Those Invaders who knew the real Captain America refused to do so. He claimed that today's world requires a tougher symbol of America to battle terrorism. Alongside the Invaders he battled Axis Mundi. When the Human Torch (Jim Hammond) was killed, most of the team disbanded.
Sent into battle against Norman Osborn's black-ops Thunderbolts team in Asgard, Walker's left arm and leg were severed by Thunderbolts leader Nuke/Scourge. Retired from active duty and fitted with a metal hook for a prosthetic hand (refusing better prostheses because he didn't want to become a cyborg like Nuke), Walker became the warden of the super-villain prison the Raft.
Back in Action
While transporting the Dark Avengers back to the prison, their transport went down and Walker and the team woke up in an alternate version of New York, divided in various territories controlled by heroes. While knocked out, June Covington who had managed to gain control over Hank Pym and had free reign in a laboratory, bonded Walker with that world's Venom Symbiote which had its higher brain functions removed. Being bonded with the symbiote restored Walker's lost limbs. 
Leading the Dark Avengers, Walker attempted to find out what had happened to New York and reality. They found out from Reed Richards that A.I.M.
were behind the alteration of reality, creating a distorted reality pocket to test potential scenarios and salvage technology created by the denizens of the pocket. The Dark Avengers managed to open a portal into the real world, where they crushed the A.I.M. agents. With the mission completed, and the Dark Avengers free, June used her powers to influence Walker into allowing them to remain free, and to lead them on further adventures.
Doctor Doom's Avengers
John joined Doctor Doom's Avengers, and battled the now-evil Scarlet Witch who arrived to Latveria to have revenge on Doctor Doom.The Avengers fought the Scarlet Witch and saved villagers trapped in the crossfire. 3D Man managed to get hold of the Witch and take her to a machine Doctor Doom had created to specifically canalize her abilities. Soon after being put into the machine, the Scarlet Witch escaped and set out to find Doom, leaving the Avengers behind. After condensing a portion of Scarlet Witch's powers into the device, Valeria Richards informed the heroes their job was done.
Prower Broker Enhancements: US Agent possesses various super human physical attributes as a result of undergoing the Power Broker's process:
Superhuman Strength: US Agent is superhumanly strong and is capable of lifting about 10 tons.
Superhuman Stamina: US Agent's muscles produce far fewer fatigue toxins than the muscles of ordinary humans during physical activity. He can exert himself at peak capacity for several hours before fatigue begins to impair him.
Superhuman Agility: US Agent's agility and bodily coordination are enhanced to levels that are beyond the human body's natural limits.
Superhuman Reflexes: US Agent's reaction time is enhanced to a level that is beyond the human body's natural limits.
Superhuman Durability: The tissues of his body are somewhat harder and more resistant to injury than the body of an ordinary human. However, he is not invulnerable and can sustain injury in many of the same ways as an ordinary human. However, he can withstand impact forces and trauma that would severely injure or kill an ordinary human with mild discomfort.