He is known for a recurring role as defrocked priest Matthew "Rickety Cricket" Mara on the comedy series It's Always Sunny in Philadelphia, for which he also writes and co-produces.
Hornsby plays defrocked priest Matthew "Rickety Cricket" Mara on the FX sitcom It's Always Sunny in Philadelphia, also serving as an executive producer and writer on the show. He appeared as Steve "The Hutch" Hutchinson on The Joe Schmo Show, and as Patrick on Six Feet Under. His first animated role was Fanboy in Fanboy & Chum Chum. He also starred in the independent film Pretty Bird.