The first issue features the Asgardian trickster god, Loki, who seeks revenge against his adopted brother, Thor. Using an illusion, Loki tricks the Hulk into destroying a railroad track, and then diverts a radio call by Rick Jones for help to Thor, whom Loki hopes will battle the Hulk. Unknown to Loki, the radio call is also answered by Ant Man, the Wasp and Iron Man. After an initial misunderstanding, the heroes unite and defeat Loki. Ant-Man points out that the five work well together and suggests they form a combined team — with the Wasp naming the group the Avengers. The original members are known as the "founding members," and courtesy of an Avengers Charter are responsible for the good name of the team. As a result, their wishes regarding the direction of the team are given additional weight and deference.

The roster changes almost immediately: by the beginning of the second issue, Ant-Man has become Giant-Man and, at the end of the issue, the Hulk leaves once he realizes how much the others fear his unstable personality. Feeling responsible, the Avengers try to locate and contain the Hulk (a recurring theme in the early years of the team), which subsequently leads them into combat with Namor the Sub-Mariner. This would result in the first major milestone in the Avengers' history - the revival and return of Captain America. Captain America joins the team eventually becoming field leader. Captain America is also given "founding member" status in the Hulk's place. The Avengers go on to fight foes such as Captain America's wartime enemy Baron Zemo, who in turn forms the Masters of Evil; the Lava Men; Kang the Conqueror; Wonder Man; Immortus; and Count Nefaria.

The next milestone came when every member but Captain America resigns and is replaced by three former villains - Hawkeye, the Scarlet Witch and Quicksilver. Although lacking the raw power of the original team, "Cap's Kooky Quartet" (as they were sometimes jokingly called) proved their worth by fighting and defeating the Swordsman; the original Power Man; Doctor Doom and Kang once again. They are soon rejoined by Henry Pym (who changes his name to Goliath), the Wasp, Hercules, the Black Knight and the Black Widow, although the last two do not obtain official membership status until later in the book's history.

Under the tenure of Roy Thomas as writer, the stories begin to focus more intently on characterization. The Black Panther joins the team, followed by the Vision. Thomas also established that the Avengers are headquartered in a New York City building called Avengers Mansion — provided courtesy of Tony Stark (Iron Man's alter ego), who also funds the Avengers through the Maria Stark Foundation, a non-profit organization. The mansion is serviced by Edwin Jarvis, the Avengers' faithful butler, and also furnished with state-of-the-art technology and defense systems, including the Avengers' primary mode of transport: the five-engined quinjets.