Mastras began writing for television in 2006 for the series The Evidence. He wrote the first season episode "Stringers". In 2007 he became a writer for the short-lived science fiction series The Dresden Files and wrote the episodes "The Boone Identity" and "The Other Dick".

Mastras joined the writing staff of the first season of Breaking Bad in 2008 as a story editor. He wrote the first season episode "Crazy Handful of Nothin'". The first season writing staff were nominated for the Writers Guild of America Award for best new series at the February 2009 ceremony. Mastras won the PEN Center USA West Literary Award for best teleply for "Crazy Handful of Nothin'". Mastras was promoted to executive story editor for the second season. He wrote the second season episodes "Grilled" and "Mandala". Mastras was nominated for the 2010 Edgar Allan Poe Award for Best Television Episode for "Grilled." The second season writing staff were nominated for the WGA award for best drama series at the February 2010 ceremony for their work on the second season. Mastras was promoted to producer for the third season and continued to write episodes. He wrote the episode "I.F.T.".

Mastras is the author of the novel Fidali's Way (Scribner 2009), a literary thriller that takes place in the war-torn Tribal Areas of Pakistan and Kashmir, where he has traveled. Mastras grew up in Boston, received his B.A. from Yale, and a J.D. degree from UCLA Law. Prior to writing, he worked as a criminal investigator for the public defender’s office, a counselor at a juvenile correctional facility, and a defense litigator in New York and Los Angeles. Mastras was awarded the ABC/Walt Disney Studios Writer's Fellowship in 2005.