Escaping the taunts of her fellow school students regarding her physical mutation, Los-Angeles born Doreen Green fled into the nearby woods where she quickly discovered her affinity for squirrels. Idolizing Iron Man, she adopted the costumed identity of Squirrel Girl in the hope of becoming his crime-fighting partner. Upon finally meeting him, she single-handedly defeated Iron Man's nemesis, the despotic Doctor Doom, overwhelming him with the squirrels under her control. Nevertheless, Iron Man declined her offer of partnership.

Squirrel Girl subsequently moved to New York City to fight crime, earning quite the reputation as Central Park’s greatest protector. There, Green was recruited into the would-be super-team the Great Lakes Avengers, helping them defeat Maelstrom and his agents, Batroc's Brigade, although during this time she had to deal with losing her most trusted squirrel companion, Monkey Joe, who was killed by the GLA’s rejected teammate, Leather Boy. In the wake of the disastrous battle against Maelstrom, Squirrel Girl adopted a new sidekick, calling her Tippy-Toe. After discovering that Squirrel Girl was a mutant, the other members found they shared this commonality, after which they changed their name to the Great Lakes X-Men (to avoid a lawsuit from the Avengers’ Maria Stark Foundation.)

With a mixture of success, pluck, and a cheery smile, Squirrel Girl has quickly become a favored ally of S.H.I.E.L.D., but she declined the opportunity to be a deputized agent, preferring her own team. Months before, however, she long wanted to join the New Warriors when they first appeared, in particular due to her obsessive crush with Speedball. So when Speedball was on tour promoting the New Warriors’ reality-TV show, Squirrel Girl rushed to the appearance, hoping he could catch a glimpse of her. He didn’t, not even when she stopped his enemy, the Bug-Eyed Voice, from exacting revenge. (Speedball did eventually learn of Squirrel Girl’s help, and he personally thanked her, with a kiss!)

Later called the Great Lakes Champions after winning the title in a poker tournament hosted by the Thing, the team recently encountered Deadpool after becoming officially-sanctioned heroes due to the Superhuman Registration Act.