Devlin appeared as an actor on numerous television shows throughout the 1980s. He also appeared in films including My Bodyguard, The Wild Life, Real Genius and Martians Go Home. Due to his youthful appearance, Devlin often played teenage characters despite being in his 20s.

Devlin was an advisor to video game company ZeniMax Media from 1999 to 2004.

Devlin gradually began writing scripts. His first produced screenplay was for Universal Soldier. He reached prominence as a writer/producer working alongside Director Roland Emmerich, with whom he teamed after appearing in Emmerich's film Moon 44. Together they cowrote and produced Stargate, the first movie to have a web site. The team then produced Independence Day and Godzilla. They split following the 2000 Mel Gibson film The Patriot, but re-teamed to film Independence Day: Resurgence in 2015. Devlin has also produced Cellular, Who Killed the Electric Car?, and Flyboys.

In 2013, Devlin said a Future crime film entitled Bad Samaritan was in development.

Devlin's directorial debut was the action sci-fi film Geostorm for Warner Bros. The film was set for a March 2016 release, later rescheduled for October 2017. It starred Gerard Butler as the lead with Jim Sturgess, Abbie Cornish, Ed Harris and Andy García. The film made $33,700,160 on a $120-million budget.