Callen got his start as an original cast member of Mad TV. He then went on to play numerous roles on the small and big screen including, Old School, Bad Santa, The Hangover, Sex and the City, Entourage, Californication, CSI, and Frasier. In 2012, he finished a two-season character arc on USA’s In Plain Sight.
Callen has people look at him and scream, “Bilson!”, since that's the character he played on several episodes of How I Met Your Mother. In 2011, he played Captain Frank Dashell, leader of the Undead Task Force, in the short-lived but critically acclaimed MTV comedy Death Valley.