He is best known for a cover version of the Tears for Fears song "Mad World", which he recorded with Gary Jules for the Donnie Darko soundtrack, and which became the 2003 UK Christmas number one.

He has since gone on to compose scores for the movies Out Cold, Nothing, Cypher, Orange County, My Suicidal Sweetheart, Me and You and Everyone We Know, The TV Set, a segment of Paris, je t'aime, Unaccompanied Minors, Bridesmaids, Walk Hard: The Dewey Cox Story and Daddy's Home among others.

He chose "Mad World" by Tears for Fears (originally released in 1982), which his childhood friend Gary Jules sang as Andrews played piano.

Working out of his custom-built backyard studio in Glendale, California, Andrews spent three months creating the score using an orchestra of obscure vintage synthesizers (a miniature hotwired Casio keyboard was unearthed at a garage sale for $10) and drum machines.

His concept was to play what he termed amateurish, emotional, naïve, magical and simple music on highly unemotional, inorganic instruments—for example, a calculator with built-in twelve-note keyboard that lends a haunting portamento melody to one of the film's motifs.

Other instruments used in the score include Andrews's modified piano (rather than hitting the strings directly, the hammers first make contact with a piece of soft felt, creating a warmer, slightly muffled tone), as well as his Moog and Vocoder synthesizers.