Third generation actor Mercer made his film debut with Wrestling with God. At six years old, he took his first bow on stage with his father's community theater group in Maryland. It wasn't until high school, while playing Tevye in Fiddler on the Roof, that he decided acting was his true calling. After graduation, Mercer joined a touring company and several other theaters in Washington D.C., before being accepted into Adelphi University's acting program. He won the Alexander-Barnes Acting Competition and received a full scholarship for four years. As a freshman at Adelphi, Mercer performed on the Senior Mainstage in plays ranging from Rodgers and Hammerstein to Shakespeare. His repertoire included French mime, dramatic verse, musicals, contemporary comedy, straight plays, and cabaret.
Mercer trained for three years at the Royal Shakespeare Company in London under renowned teachers such as Cicely Berry, Patsy Rodenberg, Sue Lefton, and others. During his time there, he was selected to perform in the British premiere of Leonard Bernstein's Mass under Bernstein's direction. Before returning to the United States, Mercer had already performed professionally with the Royal Shakespeare Company, appeared as "Mercutio" at the Arundel Festival and throughout Europe on an Arts Council Tour, led an English cast through the premiere of Yuri Trifonov's The Exchange outside what was then the Iron Curtain, and directed Insignificance for the Edinburgh Festival.
Upon his return to the US, Mercer began teaching acting, improvisation, stage combat, and verse. His wife Paula Tiso brought him to California, where he quickly gained his Screen Actors Guild credentials. In addition to classical and dramatic roles, Mercer is also in high demand for voice-overs and is part of a comedy troupe that performs regularly at Los Angeles night spots.