Martinez was born in Paris, France to a working-class family. His father was a Spanish professional boxer born in Morocco and his mother was a French secretary. He studied at the prestigious CNSAD and began his acting career in his native France in 1990, and quickly achieved a relative success. Martinez was raised Catholic.
In 1994, Martinez won a César Award as “Most Promising Actor” for his role in Un, deux, trois, soleil (1993). He first came to mainstream attention in Hollywood films with his performance opposite Diane Lane in Unfaithful (2002). He has since appeared in a television adaptation of The Roman Spring of Mrs. Stone (2003), S. W. A. T. (2003) and Taking Lives (2004). He also starred as Gabriel in the 2007 movie adaptation of Blood and Chocolate, a popular book.