The 54-year-old proved to be a thorn in the side of Adams from the very beginning, and raced to a 4-0 lead.
With the finish line in sight, Thornton started to feel the pressure, missing 11 match darts as Adams won his first set of the night.
But the Scotsman regained his composure in the following set and eventually clinched the title, adding it to an extensive list of honours that includes the World Grand Prix, UK Open and World Masters.
Thornton started strongly, breaking Adams' throw in the first set with an 89 checkout. He followed it up with a 177 and then responded to an Adams 180 with one of his own. A checkout of 44 sealed the second leg and left him one away from the opening set.
A superb average of 118 in the opening set tells you all you need to know about the Scotsman's strong start, and he capitalised on his superb form by nailing a double 20 to go 1-0 up.
Thornton managed another whitewash in the second leg as he continued to pounce on Adams' mistakes. The 65-year-old Englishman was given a great chance to break in the second set but missed bull, which Thornton swiftly punished.
With Adams already 2-0 down, he finally won his first leg of the final to take the lead in the third, but 'The Thorn' managed clinical checkouts of 68 and 116 to quickly turn the tide back in his favour. Another 68 checkout sealed the set and gave the Scotsman a 3-0 lead.
Thornton again looked dominant in the fourth set, racing to a 2-0 lead with checkouts of 68 and 20.
Adams fought back, though, and claimed a 10-dart leg before winning again with a 15-darter to set up a deciding leg.
Thornton looked momentarily flustered by the Adams comeback as he missed double 20 for a 111 checkout, but returned with a double 10 to go 4-0 up.
In the fifth set, the whitewash looked plausible as Thornton found himself 2-1 up and one leg away from the title, but nerves creeped in for the 54-year-old, who continously missed match darts as Adams claimed his first set.
But he returned confidently in the final set, holding throw in the first leg while Adams claimed an 81 checkout to level. But a clinical double 16 helped him go 2-1 up and inch closer to the title.
Adams levelled again after Thornton missed the chance for 88, but a clinical last leg left him with a double two for the title.
Thornton's win earned him a £30,000 prize, and the Scotsman was ecstatic with his performance.
"It's absolutely amazing," said Thornton. "I've got to make history twice this week. I opened the first match and I've won it - first on, last off, that will do me.
"I've been comfortable all week and felt brilliant up there.
"This is the first event of its kind and I'm very happy to be the first name on the trophy.
"Words can't describe it. I'm looking forward to the next one now, I'm enjoying my darts again."