Trainor was born and raised in San Diego, California, the son of Bill Trainor and Madelyn (née McNenly) Trainor. He studied improv at the Groundlings school in Los Angeles. Trainor also studied theatre at the University of California in Santa Barbara.
Trainor's first television role was in the MTV series Undressed as Eric. Some of his other television credits include Law & Order, Angel, and Malcolm in the Middle. His first recurring role was as Brian "the A.V. guy" on Crossing Jordan. He has also had a number of small roles in films such as Donnie Darko and Bring It On Again.
Trainor is well known for his work on the Nickelodeon television network. He had a recurring role on Drake & Josh as movie theater employee "Crazy" Steve from 2004 to 2007. He starred in iCarly as Spencer Shay, the older brother and guardian of Miranda Cosgrove's title character from 2007 to 2012. He completed the 2008 music-themed comedy Wreckless Epic, in which he stars. Trainor also starred in the animated series T.U.F.F. Puppy as the voice of title character Dudley Puppy. In 2011, Trainor starred alongside iCarly co-star Jennette McCurdy in the Nickelodeon television movie Best Player. In 2013, he starred in the short-lived show Wendell & Vinnie as Vinnie.
Trainor was involved in a World of Warcraft video series titled Project Lore, playing the Draenei shaman Goggins. He was replaced by Jeff Cannata after leaving the series in February 2009. In 2015, Trainor played Commander Michael Sullivan for both seasons of the Halo in-universe podcast Hunt the Truth.
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