Brennan credits his high school theatre director John Marquette for inspiring him to act.

[6] His early writing experience was limited to sketch pieces in high school and college plays he describes as "terrible".

[4] Brennan conceived the idea for Glee based on his own experience as a member of the Prospect High School show choir.

"[4] Brennan was inspired by these events,[6] and in August 2005, purchased a copy of Screenwriting for Dummies and wrote the first draft of Glee, initially envisioned as a film rather than a television series.

Murphy and his Nip/Tuck colleague Brad Falchuk suggested that Glee be adapted to a television show format.

Brennan, Falchuk and Murphy created Scream Queens, a comedy-horror series set on a college campus and in a hospital, that aired from September 2015 to December 2016,[11] and starred Emma Roberts, Ariana Grande, Lea Michele, Keke Palmer, Abigail Breslin,[12] and Jamie Lee Curtis.