Home-crowd favourite Rubens Barrichello, Button's Brawn GP teammate and his closest contender, started from pole but finished only eighth after a puncture, while Red Bull's Sebastian Vettel was fourth.
The fifth-place was good enough to give Button an insurmountable 17-point lead in the drivers' standings ahead of the season-ending race in Abu Dhabi on Nov. 1.
Australia's Webber took his second F1 win ahead of Robert Kubica of BMW Sauber and defending champion Lewis Hamilton of McLaren at the 4.3-kilometer (2.6-mile) Interlagos track.
Barrichello had to win and hope Button finished fourth or worse, but failed in his attempt to clinch victory in front of his home crowd for the first time in his 17-year career.
Button's triumph gives Britain back-to-back F1 titles for the first time since Graham Hill won in 1968 and Jackie Stewart in 1969. Hamilton won last year, also at the Brazilian GP.
Brawn GP also made history on Sunday, becoming the first team to win the constructors' championship in its debut season.
"It hasn't sunk in yet, it hasn't sunk in yet," team principal Ross Brawn said. "It'll take awhile."
Button started only 14th after a poor qualifying under heavy rains on Saturday, but he drove aggressively from the beginning to quickly move up the grid. He also took advantage of an eventful start in which several cars had problems ahead of him and dropped to the back of the pack.
He was up to ninth place after the safety car left the track on the third lap, and then he made bold passes to overtake the Renault of Romain Grosjean and the Williams of Kazuki Nakajima to move to seventh.
Vettel also had a good race after starting only 15th, finishing fourth, but it was not enough to keep his title hopes alive.
