Diesel made his stage debut at age seven when he appeared in the children's play Dinosaur Door, written by Barbara Garson. The play was produced at Theater for the New City in New York's Greenwich Village. His involvement in the play came about when he, his brother, and some friends had broken into the Theater for the New City's space on Jane Street with the intent to vandalize it. They were confronted by the theater's artistic director, Crystal Field, who offered them roles in the upcoming show instead of calling the police.
Diesel remained involved with the theater throughout adolescence, going on to attend the city's Hunter College, where his creative writing studies led him to begin screenwriting. He has identified himself as a "multi-faceted" actor as a result of early difficulties finding roles due to his mixed heritage.