William Fierro is a running back from La Jolla, California, who attended The Bishop's School. He joined SMU as a freshman in 2023 and redshirted that season. In 2024, his redshirt freshman year, Fierro appeared in one game. He did not see action during the 2025 redshirt sophomore season. Fierro, standing 6'0" and weighing 182 pounds, wears jersey number 25 and returns as a redshirt junior for the Mustangs.