He was nominated for a 1999 Joseph Jefferson Award for Actor in a Supporting Role in a Musical for Finian's Rainbow at the Marriott Theatre in Chicago, Illinois.
Brennan is a Mount Prospect, Illinois native. He graduated from Prospect High School in 1996 and Loyola University in 2000. Brennan was also in show choir while attending Prospect High School.
The original idea of Glee was inspired by Brennan's own experience as a member of Prospect High School's show choir. He said: "I find it interesting that there is something in everybody, a longing for something transcendent, particularly in a place like Mt. Prospect, a place that’s very suburban and normal and plain. Even in places like that, there’s this desire to shine. That’s fascinating and very funny to me, especially when people try to accomplish this through show choir – which, to me, is inherently a little ridiculous." It was supposed to be a film instead of a television series; Brennan wrote the first draft in August 2005, and completed it in the same year. The script, however, did not receive any interest from producers at that time.
Mike Novick, a friend of Brennan's and a television producer, later gave Ryan Murphy a copy of Brennan's script, as they are members of a same gym centre. Murphy himself joined show choir when he was young, and thus related to the script. Murphy and Falchuk suggested that Glee should be produced as a television show. Therefore, the three joined together and rewrote the script; the rewritten script was sent to FOX, and picked up by FOX immediately.
The three decided to write all of the episodes by themselves. Murphy and Falchuk became executive producers and directors of the show, while Brennan is a co-executive producer. Novick also became a producer of Glee.
- The Substitute
- A Very Glee Christmas
- The Sue Sylvester Shuffle
- Blame It on the Alcohol
- A Night of Neglect
- Prom Queen
† denotes episodes written with Ryan Murphy and Brad Falchuk.
- He is the narrator at the start of each episode's "So here's what you missed on Glee", and on GleeonFox's "Glee-Cap."
- He portrayed Svengoobles, a judge at Regionals in On My Way.
- He also made a cameo in Glee, Actually during Hanukkah, Oh Hanukkah.
- He made his directorial debut in the Season Three episode, Props.
- He lives in both Los Angeles and New York.
- He wanted to become an actor.