A Denver jury returned the verdict Thursday after deliberating for a day and a half.
Clark could face a sentence of life in prison. There was no immediate word on a possible appeal.
Williams, a one-time star at Fort Worth's O.D. Wyatt High School, was killed on New Year's Day 2007. Prosecutors said Clark fired the fatal shots from an SUV that pulled up beside a rented limo carrying Williams and some friends.
Prosecutors said Clark was angry over an altercation involving friends of Williams and friends of Clark in a nightclub just before the shooting.
Defense attorneys said Clark was being made a scapegoat and wasn't in the SUV.