Matty Fryatt put the Tigers ahead after just five minutes. It came after a superbly-executed quick counter-attack down the left wing. Robert Koren paced away and pulled the ball back from the byline for Fryatt to tap in an easy goal.
The Iron held their own in terms of possession but, save for Bobby Grant's sliced shot and Michael Nelson's header from Mark Duffy's corner, they failed to carve out a chance of any sort until Duffy's thumping volley was turned away by Hull goalkeeper Brad Guzan in the 31st minute.
Nelson volleyed just off target for Scunthorpe soon after and then Lee Miller mis-hit his shot inside the area following Eddie Nolan's free-kick as Hull defended their slender lead.
The visitors had soaked up everything Scunthorpe had to offer during the first half and then broke out to extend their lead five minutes into the second period.
Aaron Mclean reacted quickest when Nelson's error - trying to head the ball back to Josh Lillis from well inside the box - put Scunthorpe in trouble and broke his Tigers duck from six yards.
The home side's efforts were rewarded after 55 minutes. Hull defender Paul McShane fouled Grant out wide on the left and from Duffy's free-kick Garner headed home his first goal in his second game for the Iron.
Mclean skewed a shot from the edge of the area wide and Koren was wayward with his 25-yard shot, while Duffy saw his effort from similar distance blocked by Hull defender James Chester at the other end.
Hull defender Andy Dawson secured his side a penalty in the 73rd minute when he went down following a challenge from Bobby Grant in the penalty area.
Fryatt stepped forward to fire his fifth goal in seven games for the Tigers from the spot.
Nick Barmby, a second-half substitute for Tom Cairney, set up Mclean for Hull's fourth in the 81st minute, clipping the ball inside from the left edge of the area for the former Peterborough forward to arrow the ball into the bottom corner from 12 yards.
Michael Collins had a shot blocked as Scunthorpe refused to roll over, but Fryatt slammed home his hat-trick goal in the 89th minute following Belaid's corner.