Marcell was born on the Caribbean Island of St. Lucia and moved with his family to England when he was 5. He studied at Sheffield University in Yorkshire and graduated from the Central School of Speech and Dance.

As a member of the Royal Shakespeare Company, Marcell appeared in "Twelfth Night," "Julius Ceasar," "Othello," "A Midsummer Night's Dream," "Anthony and Cleopatra" and "Macbeth." Some of his other London stage credits include "Joe Turner's Come and Gone," "Black Star," "Split Second," "Sherlock Holmes," "Motor Car," "One Fine Day" and "A Dying Business."