He is best known for his roles as "Crazy" Steve in Drake & Josh and Spencer Shay in the teen sitcom iCarly and its revival series of the same name, winning three Kids' Choice Awards for his performances.

[5] He studied drama at the University of California, Santa Barbara graduating with a Bachelor of Fine Arts degree.

He has had several small roles in films such as the science fiction drama Donnie Darko and the cheerleader comedy Bring It On Again.

Trainor was involved in a World of Warcraft video series titled Project Lore, playing the Draenei shaman Goggins.

He played Commander Michael Sullivan for both seasons of the Halo in-universe podcast Hunt the Truth.

Since 2018, Trainor and Mike O'Gorman have starred in a comedy Web series called The Porch, which can be found on YouTube.

In 2018, Trainor starred in the film Cover Versions written and directed by award-winning filmmaker Todd Berger.

Parts of the film were shot on location during the KAABOO Del Mar music festival (in coastal northern San Diego County).