The Millers got off to an absolutely blistering start with a goal after only 55 seconds, with Michael O'Connor firing home from 25 yards.
Bradford responded and Luke Oliver's shot was saved by Scott Shearer before Kyel Reid blasted the lose ball across the face of goal.
Bradford continued to dominate, but Rotherham doubled their lead 24 minutes in when Ben Pringle broke down the left and his cross found Gareth Evans on the right side of the area and he fired past Matt Duke.
Ten minutes later former Derby midfielder Pringle picked up the ball deep in his own half and ran through the heart of the City defence before beating Duke with a shot from the edge of the area.
Rotherham should have had a fourth after 65 minutes but Evans shot over an empty net after Gary Jones cleared off the line from Kayode Odejayi. But the fourth goal did come five minutes from the end when Evans got his second of the afternoon, firing home from the corner of the box.