His plays include The Cider House Rules, Flaubert's Latest, Hyde In Hollywood, An Imaginary Life, QED, Rise and Rise of Daniel Rocket, Romance Language, Scooter Thomas Makes It To The Top Of The World, and Sorrows of Stephen.

Parnell is also noted for co-writing the "In This White House" episode of the West Wing series with Aaron Sorkin, and the book And Tango Makes Three with Justin Richardson.

His latest play Trumpery, which revolves around the mystery of Charles Darwin, was performed by the Atlantic Theatre Company in November of 2007.