The teenager raced to a 3-0 lead in a little over 30 minutes, drawing first blood in a nervy opening set before holding off a 104 average from Humphries to double his advantage.
Littler, knocked out in the first round last year, came agonisingly close to a piece of history in the third, missing the bull for a double-start nine-darter, only to follow it up with a 104 checkout to make it 3-0.
Humphries produced flashes of brilliance, with finishes of 149, 110 and a superb 154, but could not stem the tide. Littler sealed the fourth set despite a late fightback, fired an 11-darter in the sixth to restore his four-set cushion and ultimately wrapped up the title with combinations of 98 and 111 and a 14-darter on the tops, claiming a seventh major PDC crown.
“This is not the easiest tournament to win, and this week has been so tough,” said Littler. “Now I’ve picked the trophy up, I can tick it off, and there aren’t many left.
“I learnt from the World Matchplay that I have to hit the big outshots and big scores when Luke puts me under pressure, and I think I did that very well tonight. He was always behind me, so I couldn’t step off the gas.”
Littler’s triumph followed impressive wins over Gian van Veen, Mike De Decker, Gerwyn Price and Jonny Clayton earlier in the week, further underlining his status as the sport’s most formidable rising force.
The Warrington prodigy has made no secret of his ambition to replace Humphries as world No. 1, and his Leicester heroics move him to within £70,000 of his World Cup partner. “It’s not a lot of prize money (£120,000), especially given what we play for,” he said. “Luke will know now that I’m on his back -- I’m on to him.”
Humphries, meanwhile, was left to reflect on another near miss, denied a second Grand Prix crown despite becoming only the third player after Phil Taylor and Michael van Gerwen to reach three consecutive finals.
The world No. 1 had overcome Nathan Aspinall, Krzysztof Ratajski, Cameron Menzies and Danny Noppert en route to the decider but could not keep pace with Littler’s relentless scoring.
“It’s hard to take; it’s disappointing. Luke is so clinical,” Humphries said. “In the first four sets, I missed so many darts to get in. Double 16 was my friend for the last three years, and tonight it wasn’t.
“Luke is amazing. I tried my best, but he ran away with it in the end, and his class shone through. I’ll be back. I’ve got many more battles with him ahead, but I’ll need to be better for them.”
The defeat served as a reminder to Humphries of the scale of the challenge now facing him, and of the unstoppable force emerging on his shoulder. For Littler, already a winner at the UK Open and World Matchplay this year, the victory marks yet another milestone in a season of extraordinary achievement.