Williams dove to snatch the ball and dragged his left foot along the turf before falling out of bounds.
Officials initially ruled Williams' catch incomplete, but overturned it after a review.
The pass traveled 25 yards for a first down. Williams capped the drive with a nine-yard touchdown pass to give the Gophers a 21-7 lead.
He added another touchdown grab before the half to put Minnesota up 35-7.