Diesel's first film role was an uncredited appearance in the 1990 film Awakenings. He then produced, directed and starred in the 1994 short film Multi-Facial, a short semi-autobiographical film which follows a struggling actor stuck in the audition process, because he is regarded as either "too black" or "too white", or not black or white enough. He made his first feature-length film, 1997's Strays, an urban drama in which he was self-cast as a gang boss whose love for a woman inspires him to try to change his ways. Written, directed and produced by Diesel, the film was selected for competition at the 1997 Sundance Festival, leading to an MTV deal to turn it into a series.

He was then cast in Steven Spielberg's 1998 Oscar-winning film Saving Private Ryan on the poignancy of his performance in Multi-Facial. He then earned critical acclaim for voice work as the title character in the animation film The Iron Giant (1999). He followed it up with a major role in Boiler Room (2000)