In 1991, Walker made his feature debut in "Young Soul Rebels", a period piece about London's latter-day soul movement. He was next featured in "Shopping", a 1994 drama chronicling a pair of outlaws (Jude Law and Sadie Frost) who crash stolen cars into shop windows for thrills. Walker returned to series television the following year, playing convict Snoopy Oswald in "The Governor" (1995-96), an ITV prison series produced and directed by Lynda LaPlante and starring Janet McTeer as a female warden. He reteamed with LaPlante in "Supply & Demand" (ITV) in 1997, playing a veteran detective who goes undercover to bust a drug operation.

Work on "Oz" brought Walker to the USA in 1997, and he brought to the role of Said a quiet grace and power, as well as a flawless American accent. Working with producer Tom Fontana, Walker created a fully realized character who showed his human frailties and spiritual sincerity even while engaging in self-righteous grandstanding. Between the third and fourth seasons of "Oz", Walker was featured in "Homicide: The Movie", the 2000 TV-movie reunion of Fontana's acclaimed NBC series. Here he impressed with a pivotal role as a news cameraman who has lost a son to drug abuse and is driven over the edge, shooting a mayoral candidate pushing for drug legalization. Later that year, he was featured in "Once in the Life", a drug-themed drama about two ne'er-do-well brothers, written, produced and directed by as well as starring Laurence Fishburne.