William Joseph McCormack Jr. was born on January 13, 1974 in Plainfield, New Jersey, to william Sr. and Norah Magdalene (née Ross) McCormack. He has two older sisters, Mary, an Actress, and Bridget, a Michigan Supreme Court Justice and former University of Michigan law professor. He graduated from Trinity College in Hartford, Connecticut.
McCormack's key television roles include Jason La Penna on The Sopranos (1999–2001). More recently, he played the role of Leo Spiller, brother of lead character Lucy Spiller (Courteney Cox), in the FX series Dirt. He also played a recurring role as the FBI agent in the television drama In Plain Sight, which stars his older sister, Mary.
In 1996, McCormack appeared on Broadway at the Criterion Center Stage Right as a member of the ensemble in a revival of the drama play Summer and Smoke (1948) by Tennessee Williams, produced by The Roundabout Theatre Company. He has also appeared in two Off-Broadway theatre productions, Chinese Friends and The Long Christmas Ride Home. On January 18, 2018, it was announced that Toy Story 4 would be written by Stephany Folsom, replacing the original Writers, Rashida Jones and McCormack, who withdrew from the project in November 2017.
