The early stages were evenly matched, with both players producing strong visits but also showing signs of caution.
Littler held throw to open the contest and established an initial break for a 2-0 lead. Humphries responded with a break of his own, settling after a hesitant start, yet the overall rhythm of the match tilted in Littler's favour as he reached finishes with greater regularity.
The first significant shift arrived when the players reached 5-5. Humphries then produced his best run of the evening, claiming three straight legs for a 7-5 advantage and momentarily disrupting Littler's control.
His scoring tightened, and he applied pressure more effectively, hinting at a possible momentum swing.
Littler halted that surge immediately. A tidy 81 checkout steadied his position before he accelerated again.
A standout moment came when he completed a 167 finish immediately after Humphries had failed to convert a 170 attempt. That checkout reasserted Littler's authority and signalled the beginning of a more decisive stretch in the match.
From there, Littler's scoring power and timing became the defining features of the contest.
He moved 10-9 ahead after producing consecutive 180s and, despite occasional hesitation on doubles, maintained the initiative.
A clinical 160 checkout then pushed him further clear, and Humphries struggled to mount a meaningful response as the gap opened to 12-9.
Littler too relentless
Humphries continued to fight, reducing the deficit at 12-10, but could not consistently reach finishes early enough to threaten.
A missed attempt at 66 allowed Littler to restore control before he extended his advantage to 15-10 with another composed leg on his own throw. Humphries held once more, yet Littler closed out the match in the next leg with a measured three-dart clearance on 96.
Littler reached checkout ranges more efficiently, delivered heavier scoring across longer sequences and maintained greater clarity during decisive visits.
"I've got to say it was a tough job getting up this morning," a beaming Littler told Sky Sports.
"No one likes playing in the afternoon, but you have to get through it. Once I got over the line, I knew I was number one.
"Luke played amazingly. It was a weird game. Glad to go back-to-back. We're never going to run away with anything. Do the hard work up until the fina,l but then relax and let them go.
"The 160 to go in the break at 11-9. I was gone for the first six darts of that leg. He tried to hit the 170, and he said 'That's what happens when you try and hit it."
Humphries 'declaring war'
Humphries produced scattered bursts of high scoring but lacked the sustained precision required to shift the match back in his direction.
"I am really proud," the beaten Humphries said afterwards.
"It is three finals on the trot I've lost. I'm ready for the Worlds now. I am going to win the Worlds.
"I am declaring war on him. I'm playing good. Tiredness just crept in at the end there. We're going to go to war at the World Championships, we'll see who is the one there.
"I said to him he deserves everything he gets. He's a massive advocate for darts. I've never really felt like world number one, I've never felt like I've got the attention I deserve. He's such an incredible player."
Humphries remains a consistent top-level competitor, although on this occasion he was outpaced by a player operating with sharper timing and greater conviction on the big moments.
Littler ultimately lifts the Eric Bristow Trophy after a display that combined maturity, power and poise, reinforcing his emergence as a central figure in the current darts landscape.
Read more: Littler cements world number one spot with big Grand Slam win over Noppert
A stunning season for Littler
Earlier in the evening, Littler officially claimed the world No. 1 ranking after an emphatic 16-9 semi-final win over Danny Noppert, securing his place in the final and setting a new benchmark in the sport.
At 18 years and 299 days old, Littler becomes the youngest PDC world No. 1 on record, surpassing Michael van Gerwen's previous mark of 24 years and 251 days set in 2014.
Since lifting his first world title at Alexandra Palace in January, Littler has added the World Matchplay, World Grand Prix and UK Open to a catalogue of trophies that already includes wins at the Australian Darts Masters, New Zealand Darts Masters, Belgian Darts Open, Flanders Darts Trophy and Players Championship 32.
Before the Grand Slam, Humphries held a £72,000 cushion over him in the rankings, yet Littler's return to the final has swung that gap to £81,500 in his favour.
Sunday's win stretched the margin to £161,500 and crystallised the power shift that has been brewing for months.