The striker was fortunate to get the first goal of the game as a clearance hit him and rebounded into the Preston goal in the latter stages of the first half.
Hull then made the points safe with a superb Curtis Davies volley late in the second half as the Tigers maintained their good start to the season.
Hull captain Michael Dawson did well to challenge Joe Garner after the ball had been played into the Tigers area as the visitors started quickly.
Chris Humphrey then tested Allan McGregor with a left foot shot after cutting in off the right wing and the Hull keeper pushed the ball out for a corner, diving low to his right.
Hernandez had Hull’s first shot after 10 minutes as he and David Meyler led a breakaway from a corner but the striker fired well over.
The same player then tried a bicycle kick after again being set up by Meyler but the effort lacked power.
With just under half an hour gone Moses Odubajo cut in from the right and hit a left foot shot which slid past the post.
Meyler fed Odubajo on the right but his cross was too deep as the home side started to dominate.
Preston repelled the next attack down the right but from the resulting corner Hull took the lead after 36 minutes.
Preston failed to clear Sam Clucas’ flag kick to the far post and when Neil Kilkenny kicked the ball off the goal line it hit Hernandez and ricocheted into the goal.
Hernandez fancied a second goal from a free-kick just outside the area but hit his shot well over the bar.
Hernandez should have had his second when a shot from Dawson was deflected off a defender into his path but his shot from close range was straight at keeper Jordan Pickford, who saved with his legs.
Daniel Johnson hit a close-range shot which Dawson blocked and McGregor back-tracked to claim the loose ball early in the second half.
Garner then headed over as Simon Grayson’s side again started at pace.
Davies headed over from a Clucas free-kick 15 minutes into the second half as the Tigers finally put in a threat on the Preston goal.
Shaun Maloney made his debut with 20 minutes to go just two days after signing from MLS side Chicago Fire and was involved in a challenge with Pickford for a poor back pass which the keeper won.
But with nine minutes to go Davies claimed Hull’s second as he volleyed into the roof of the net from Maloney’s out-swinging corner on the left to secure a deserved victory.