He is a grandson of Jerry Gaghan, a newspaper columnist and drama critic for Variety and the Philadelphia Daily News.

[5] Gaghan wrote in a 2001 article in Newsweek, "I also wanted to be a writer, like my grandfather, who carried a card in his wallet that read, "If you find me, call my son [my father] at this number..."[6] In his final days of high school before graduation, Gaghan was expelled for driving a go-cart through the halls of the school.

[8] He was a student on the Fall 1986 Voyage of the "Semester at Sea" Study Abroad Program where he attended classes on board the SS Universe and sailed around the world.

Other writing credits include Rules of Engagement (2000), Havoc (2005), and The Alamo (2004), as well as a handful of episodes of various television series.

In his television writing career, he won an Emmy Award for co-writing a NYPD Blue episode entitled Where's Swaldo, in 1997.

He has also been hired by Warner Bros. to write the screenplay of the Dead Spy Running franchise written by author Jon Stock.