In a race won by 20-year-old Dutch prodigy Max Verstappen, in a Red Bull, the 32-year-old Mercedes driver finished ninth to cement his place as his country's most successful driver of all time.
Vettel, the only man who could have delayed the seemingly inevitable, ended up fourth after starting on pole position and then dropping to 19th following a pitstop to replace a broken front wing.
Hamilton has an unassailable lead of 56 points with two races, worth a total of 50, remaining in Brazil and Abu Dhabi.
"It doesn't feel real. That's not the kind of race that you want but I never gave up. I kept going right to the end," said a jubilant Hamilton, the British flag proudly draped over his shoulders.
He had raised both hands to his helmet as he took the chequered flag, with the crowd rising to applaud.
"Lewis has done a superb job all year and deserves to win the title," said Vettel. "Congratulations to him. It is his day."
Sunday's race was both the best and worst of afternoons for the sport's biggest star, who ended up sprinting down the pitlane chased by fans.
A winner nine times this season, including five of the six before Mexico, ninth was his lowest placing of the campaign and he did it despite having a badly damaged car at a track where overtaking is difficult enough anyway.
Hamilton's team mate Valtteri Bottas finished second at the Autodromo Hermanos Rodriguez with Ferrari's Kimi Raikkonen third.
AGGRESSIVE MOVE
Vettel had needed to be in the top two to have any chance of taking the title fight down to Brazil in two weeks' time but his already slim hopes seemed to have disappeared within seconds of the start.
Verstappen, with nothing to lose and everything to gain from his front row position, seized the lead with an aggressive move through the opening right-left-right corners and the Red Bull bumping wheels with Vettel as he went through.
Hamilton, starting in third place, tried to follow Verstappen but the Ferrari's front wing sliced Hamilton's rear right tyre as they made contact at turn three.
"Did he hit me deliberately?" asked Hamilton over the radio, limping back to the pits and fully aware that Vettel's only real hope of getting back into the reckoning would be if the Briton went out.
"Not sure, Lewis," his race engineer Peter Bonnington said in reply. It looked far from deliberate and stewards swiftly decided that no further investigation of the incident was necessary.
Vettel pitted while Hamilton, who had started the day 66 points clear of his rival, had a longer stop while mechanics inspected his car for further damage.
The incident robbed the crowd of the prospect of a real duel between the two contenders, who will both be four-times champions when next season starts, but they still provided thrills as they fought back.
HIGH FIVE
Hamilton, who had a thrilling wheel-to-wheel tussle with former McLaren team mate Fernando Alonso in the latter stages that could have cost him dear, had hoped to celebrate by spraying the winner's champagne from the top of the podium.
Instead, there was the considerable consolation of being one of only five men -- Germany's Michael Schumacher, Argentina's Juan Manuel Fangio, France's Alain Prost and Vettel -- to win four titles or more since the championship started in 1950.
Prost and Vettel both have four, with the late Fangio on five and Schumacher seven.
Hamilton's tally of titles took him above fellow-Briton Jackie Stewart and also his late Brazilian idol Ayrton Senna in the all-time lists.
"An unusual way to be world champion but you are world champion very simple. Nobody cares how you do it," said Mercedes' non-executive chairman Niki Lauda, himself a triple champion.
"Who cares? It's about the result," said team boss Toto Wolff, when asked how it had felt to see Hamilton lapped by Verstappen. "He was lapped because he was crashed into."
The victory was the third of Verstappen's career and second of the season, cementing the youngster's position as the rising star of the sport.
France's Esteban Ocon was fifth for Force India, with his Mexican team mate Sergio Perez seventh and behind the Williams of Canadian rookie Lance Stroll.
Danish driver Kevin Magnussen was eighth for Haas with Alonso taking the final point.
(Reporting by Alan Baldwin, editing by Pritha Sarkar)
source: news.abs-cbn.com
