Marshall made an early career appearance on the police series The Bill but it was in 2000 that his major breakthrough role came as Nick Harper in the BBC sitcom My Family. In 2003, he appeared in the film Love Actually as Colin Frissell, an Englishman who goes to Milwaukee, Wisconsin, in the United States to find love. In 2004, Marshall appeared as DS Luke Stone in the police drama series Murder City. From 2005 to 2011, Marshall appeared on TV and in print for BT Retail adverts, where he played the character Adam; with Esther Hall portraying his character's eventual wife Jane. He also played Dave in the BBC comedy series Citizen Khan in 2012.