He is the creator of the animated comedies Solar Opposites (with Justin Roiland) and Star Trek: Lower Decks.

He was one of the first writers hired for the series, as he had met co-creator Justin Roiland while working at 20th Century Fox Animation.

McMahan described himself as "kind of the sci-fi guy" of the staff, and was promoted to showrunner for the show's fourth season, though he left during production to work on other projects.

[3][4] In 2018, he won the Primetime Emmy Award for Outstanding Animated Program for his work as a supervising producer on the episode "Pickle Rick".

On August 28, 2018, Hulu announced it had ordered an animated comedy Solar Opposites created by McMahan and Roiland.