Jack King scored his first goal of 2013 as Preston increased Scunthorpe's fears of being relegated from League One.
Lee Holmes put improving North End ahead, slotting Stuart Beavon's pass under Eirik Holmen Johansen, before Nicky Wroe pounced on the on-loan keeper's poor clearance to make it 2-0.
King rifled against the post late on but nodded in Jeffrey Monakana's cross soon afterwards to seal the win.
The defeat left the Iron in the bottom four zone on goal difference.
Scunthorpe are level on points with Oldham but the Latics have two games in hand.
VIEWS FROM THE DRESSING ROOM
Preston North End manager Simon Grayson told BBC Radio Lancashire:
"As a manager, you've got to look at the table and until you're mathematically safe you have to keep working hard and keep producing performances.
"We've got a duty to finish the season in a positive manner.
"In the first half we were comfortable without creating too many opportunities. We got the first goal and from then on we were in control, but you always need that second goal. That was a big moment for us."
Scunthorpe United manager Brian Laws told BBC Radio Humberside:
"I can't complain about the result today. Preston were better than us on the day.
"They were avoidable goals, the first one in particular, where we've not tracked a runner, and then he's had the run of the box and an easy task to put the ball in the back of the net.
"The second goal was a comedy of errors. The goalkeepers made a mistake, he's not cleared the ball and it's gone in the back of an empty net. It knocked the stuffing out of everybody."