The Avengers were formed after Captain America was found and thawed out by S.H.I.E.L.D., and first saw action in thwarting an attempted alien invasion by the Chitari.
The team would later meet Superman during a mission Latveria in neutralizing stolen missiles that were to be use in a live demonstration on a naval base in New York as part of a weapons sale by Doctor Doom, they were shot down by Latverian anti-aircraft fire and saved by the Man of Steel. The Avengers were grateful for the young hero's help and accepted his help on their mission, which they succeeded in neutralizing the missiles. In gratitude to Superman's help, Captain America presented him a comm-unit to contact the Avengers if he and his friends needed urgent help.
The Avengers would from time and again assist Superman and by extent the X-Men. This cooperation includes destroying a Project Rebirth device that was stolen by Magneto, and battling the Friends of Humanity after mutants were exposed to the public.
In the aftermath of mutantkind becoming public that include the formation of the Justice League, the Avengers shared on mutual grounds with the new superhero team in spite of the American government's mixed opinion on said team.