McGrath attended Harvard University, and his first job as a writer was at Saturday Night Live in 1992. He also worked on The Simpsons as a writer, composer, story editor, consulting producer and executive story consultant. In 1992, McGrath was nominated for a shared Primetime Emmy Awards, and in 1997 he won a shared Primetime Emmy Award for Outstanding Animated Program" for work on The Simpsons. In 2009 he was nominated for a WGA Award for work on King of the Hill.
McGrath's most recent work was on Gravity Falls, where he worked as a story editor during season one.