Mastbaum acted in and produced the 2005 Sundance Film Festival official competition film, Ellie Parker, directed by Scott Coffey.

Clay's Way

Mastbaum's first novel, 2004's Clay's Way, won a Lambda Literary Award.

Us Ones In Between

Mastbaum's second novel, Us Ones In Between, published by Running Press and released in May 2008, centers on depressed art school graduate Kurt Smith, who fantasizes about pushing boys in front of subway trains. The title is taken from the song "Us Ones In Between," written by Spencer Krug and performed by the band Sunset Rubdown. The novel was a finalist for the 2008 Ferro-Grumley Award.

Mastbaum edited the anthology Cool Thing: The Best New Gay Fiction by Young American Writers, released by Running Press on November 10, 2008.