William Mark McCullough earned a political science and theater degree from Mercer University before earning a law degree from American University in Washington, D.C.  Prior to his career in entertainment, Mark worked in politics and law on Capitol Hill.  A serious car accident while on a trip to Nicaragua prompted him to change his career path.

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Mark's breakout role was in the Doug Liman biopic, American Made, playing Tom Cruise's co-pilot and fellow drug smuggler.  His most recent work on the big screen is in the Marvel film, Captain America: Brave New World, playing Commander Dennis Dunphy who is a long time friend of the titular character played by Anthony Mackie.  Captain America asks Dunphy for a favor that puts him on a collision course with The Leader, played by Tim Blake Nelson.  In the Marvel comics, Dennis Dunphy gains superpowers and becomes Demolition Man.

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Mark has acted opposite numerous A-list talent including Nicolas Cage, Channing Tatum, Mahershala Ali and Glenn Close.  He has been directed by such luminaries as Ron Howard, Steven Soderbergh and Gary Ross.

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He has guest starred in numerous TV shows including Ballard (Prime), Manhunt (Apple TV), Quantum Leap (NBC), Sweet Magnolias (Netflix), The Resident (Fox), The Walking Dead (AMC), Cobra Kai (Netflix), L.A.'s Finest (Fox) and The Gifted (Fox).

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Mark is an award-winning actor, director and producer.  He wrote and directed the supernatural horror film, A Savannah Haunting, which won Best Film awards at notable film festivals around the world.  He is the co-president of the production company, Fort Argyle Films.