As part of his Marvel work, he co-created the character Gorr the God Butcher and introduced the concept of Jane Foster becoming Thor.

[2][3][4] His cousin, Gustav Hasford, who wrote the semi-autobiographical novel The Short-Timers (1979), on which the feature film Full Metal Jacket (1987) was based, was a large influence on Aaron.

[7] The story, which was published in Wolverine #175 (June 2002), gave him the opportunity to pitch subsequent ideas to editors.

[5] In 2006, Aaron made a blind submission to DC Comics's imprint Vertigo, which became his first major work, the Vietnam War story The Other Side.

[5][9] Following this, Vertigo asked him to pitch other ideas, which led to Scalped, a creator-owned series with artist R. M. Guéra set on the fictional Prairie Rose Indian Reservation.

[11] Later that year, Marvel editor Axel Alonso, who was impressed by The Other Side and Scalped, hired Aaron to write issues of Wolverine, Black Panther and eventually, an extended run on Ghost Rider that began in April 2008.

[29] In August 2023, it was announced that Aaron would be writing a project for DC called Batman: Off-World, a six-issue limited series drawn by artist Doug Mahnke and inker Jamie Mendoza.

[31] In October 2023, it was announced that Aaron's next DC project would be a three-issue story arc on Action Comics focusing on the character Bizarro to be drawn by artist John Timms.