Timothy originally trained to be a modern dancer. He had a scholarship with The Nancy Spanier Dance Theater in Boulder, Colorado before a knee injury ended his career. He was introduced to improvisational comedy while attending a workshop at a club in Denver and began taking classes there. Shortly thereafter, he was invited to join a comedy troupe called Head Games, which he would later go on to direct. While performing weekly shows in Boulder and Denver, he signed with his first agent. He also started studying acting at Bill Howey's studio, while regularly booking local and regional commercials as an on-camera and voice over talent. At Howey's prompting, he moved to Los Angeles in 1999 and began taking classes at the Groundlings Theater. While moving up through the Groundlings' school, he also studied regularly at Improv-Olympic West and The Second City. In 2001, he was promoted to the Groundlings' Sunday Company. Here, he honed his comedic writing and acting skills for the next year and a half, performing sketch and improv every Sunday night. In 2003 he was voted into the Groundlings' main company. He then reconnected with Howey, who had moved his studios to LA, and took up scene-study with him over the next decade. During that time, he has worked steadily in both comedic and dramatic roles and in numerous national commercials. Brennen wrote and acted in fifteen main company shows and countless improv performances in his twelve year career at the Groundlings. He has also been a go-to for sketches on JIMMY KIMMEL LIVE, making over 25 appearances. In 2016, he gave up his spot at The Groundlings and relocated to Chicago.