Thaddeus Ross served in the Vietnam War as a Green Beret, receiving his Combat Infantryman, Master Parachutist, and Air Assault badges before the age of 27. His natural leadership abilities eventually saw him rise to the rank of Lieutenant General. At some point he married and had a daughter, Elizabeth "Betty" Ross. His wife died when Betty was a teenager, and Ross threw himself into his work, becoming involved in increasingly classified research projects. One of these projects was an attempt to recreate the Super Soldier serum that created Captain America. While Howard Stark had been working on a revised formula, he was assassinated by HYDRA before he could finish it. Ross recruited Betty, now a biology professor at Culver University, and her boyfriend Dr. Bruce Banner into the project, but led them to believe the project was devoted to improving the combat effectiveness of soldiers and making them more resistant to radioactive and chemical weapons. Banner turned himself into a test subject, bombarding himself with pulses of gamma radiation, which had the side-effect of turning him into a hulking green humanoid whenever high amounts of adrenaline surged through his body. Ross confiscated his daughter's notes as Banner was forced on the run. Ross spent the next five years hunting Banner and his monstrous alter-ego, dubbed "the Hulk."
During a disastrous encounter with the Hulk and the "Hulkbuster Task Force", Ross is nevertheless impressed by the performance of the squad leader, Captain Emil Blonsky. He notes that Blonsky is pushing 40 and should be driving a desk, but he has refused promotions. Ross offers to "upgrade" Blonsky with a version of Super Soldier serum derived from Banner's research, which Blonsky accepts. However, Blonsky soon develops the ability to transform into a gamma-enhanced monstrosity, the Abomination, capable of rivaling the Hulk in size and power. Drunk with power, Blonsky goes rogue, and Banner and Ross must join forces to stop the renegade. Once Blonsky is captured, however, Banner is once again on the run. While Ross is at a bar watching news reports about concurrent events, he receives a visit from Tony Stark, who tells him about the Avengers Initiative.
Ross has a heart attack while playing a round of golf, prompting his retirement from the military. By the time of the Sokovia Incident, the President of the United States has appointed Ross his Secretary of State. Ross meets with the currently active members of the Avengers (Thor and the Hulk were off-world) and shows them the collateral damage from their previous battles. Increasing public outcry has called for a higher level of government oversight. This has prompted the United Nations to craft a series of guidelines for the use of enhanced individuals in combat situations, called the Sokovia Accords. While Iron Man favors this oversight, Captain America is leery, creating a schism within the ranks. When the conference discussing the accords is bombed, Cap's old friend Sgt. James Barnes appears to be the guilty party. Convinced that Barnes has been framed, Cap goes rogue to clear his friend's name. Ross is eager to declare them traitors, but Iron Man appeases him by promising to bring them in peacefully. The end result is massive fight between both factions of Avengers, with most of Cap's supporters being captured. Ross has them brought to a high security facility called "The Raft," something that Iron Man expresses a degree of discomfort with. Cap is eventually able to exonerate Barnes and free his allies, but they are forced on the run, with Ross demanding their recapture. Two years later, when Thanos threatens the world in search of the Infinity Stones, Ross is still angrily demanding to War Machine that the "rogue" Avengers be arrested. When Thanos acquires the stones and rewrites reality as he sees fit, Ross is among the half of the population that gets "blipped out of existence." He is restored to life five years later after the Avengers defeat Thanos, and is last seen attending the funeral for Iron Man, who gave his life to put down the Mad Titan for good.
- "As far as I'm concerned, that man's whole body is property of the U.S. army."
- ―Ross on the Hulk
- "In the past four years, you've operated with unlimited power and no supervision. That's an arrangement the governments of the world can no longer tolerate."
- ―Captain America: Civil War
- "Tell me, Captain, do you know where Thor and Banner are right now? 'Cause you can bet if I misplaced a couple of 30 megaton warheads, there'd be consequences."
- ―Ross inquiring about the whereabouts of Thor and the Hulk