He is best known for a recurring role as defrocked priest Matthew "Rickety Cricket" Mara on the FX comedy series It's Always Sunny in Philadelphia, for which he also writes and produces.

Hornsby plays defrocked priest Matthew "Rickety Cricket" Mara on the FX comedy series It's Always Sunny in Philadelphia, also serving as an executive producer and writer on the show.

Hornsby appeared in Pearl Harbor, Minority Report, and Flags of Our Fathers (in which he played USMC photographer Louis R. Lowery).

Hornsby co-wrote, designed characters, and lent his voice to FX's cartoon Unsupervised, which lasted for one season.

[6] Hornsby portrays executive producer David Brittlesbee in Mythic Quest, a comedy web television series starring Rob McElhenney for Apple TV+.