Mercer was born in Oldham, Lancashire. On leaving school Mercer trained as an electrical Engineer but decided to become an actor when he became an assistant stage manager at the Oldham Coliseum in 1979. His first television appearance was as a butcher in the film Blue Money in 1985.

His television and film acting credits include leading roles in Starting Out, (1982), a series made by ATV for schools and written by Grazyna Monvid, Coronation Street (as Gary Mallett and in 1987, Pete Jackson), Shackleton (with Kenneth Branagh), Heartbeat, The Monocled Mutineer, A Touch of Frost, Cracker, Common As Muck, Peak Practice, New Tricks and Master and Commander: The Far Side of the World (with Russell Crowe). In 2009 he appeared in an episode of Doctors and two episodes of Waking the Dead. He appeared as Blackbeard's chief zombie henchman in Pirates of the Caribbean: On Stranger Tides. During August and September 2015 he returned to Doctors playing the recurring role of Andy Weston in an ongoing sub-plot.

In 2007 he completed three years of study at Northumbria University, gaining a degree in English and Art History. In May 2011 he was convicted of benefit fraud, having falsely claimed £2,300 in council tax benefit despite being in work at the time. He was fined £165 with a £15 victim surcharge and £100 costs. Mercer lives in Northumberland with his two daughters, Scarlett Rae (1997) and Ruby Mae (2000).