McCusker grew up in Bowie, Maryland. He graduated college with a degree in journalism and spent several years writing copy for a local publisher. From the late 1970s, he began writing sketches and plays for his church, Grace Baptist, many of which were published and are still in print. Among his most popular plays are "Catacombs" and "First Church of Pete's Garage".

In 1985, McCusker moved to California to write for Continental Singers and their touring drama group The Jeremiah People. In 1987, he was invited by Focus on the Family to help develop a radio show for kids, which later became Adventures in Odyssey. He still consults on the show's scripts and writes them on occasion. He has also written 18 tie-in novels, including the "Passages" series. He won a Peabody Award for his work on Bonhoeffer: The Cost of Freedom.

In the late 1990s, Paul developed Focus on the Family's Radio Theatre. He has also dramatized C.S. Lewis's The Chronicles of Narnia, as well as A Christmas Carol, Les Misérables, Amazing Grace, the Father Gilbert Mysteries and The Screwtape Letters. Paul also writes novels, The Mill House and Epiphany being perhaps the best-known, and TSI: The Gabon Virus (2009) his most recent, which was co-written with Dr. Walt Larimore.

Paul McCusker now lives in Colorado Springs, Colorado with his wife Elizabeth and two children; Tommy and Ellie. In 2007 he converted to Catholicism.