The teenager raced to a 3-0 lead after a little over 30 minutes, putting Humphries immediately on the back foot despite a higher three-dart average.
Littler, knocked out in the first round last year, then sealed the fourth set ruthlessly, holding off a late fightback from Humphries in the deciding fifth leg.
Despite improvement by the world No. 1 in Leicester, Littler kept his composure to claim two of the following three sets for a crushing triumph -- his seventh major PDC title.
"I'm very happy to have that one ticked off," Littler told Sky Sports after lifting the World Grand Prix title. "It's not the easiest tournament to win -- this week has been so tough. Now I've picked the trophy up, that's another one done; there aren't many left.
"It's certainly not a lot of prize money (£120,000), especially for what we guys play for. Luke will know I'm on his back now -- I'm onto him," he said of Humphries.