The two title rivals collided and spun off on the first lap -- di Grassi plowing into the back of Renault eDams driver at turn 3 -- before both men limped back to the pit lane for repairs.
With both drivers out of contention for podium places it was a duel to see who could set the fastest lap of the race and pick up the two points that would seal the title.
Di Grassi had started the day three points ahead on 153 points but Buemi leveled things up after picking up three points for setting the fastest lap in qualifying.
The Brazilian, who started third on the grid, had lamented his ABT Schaeffler car's lack of pace compared with the Renault in the build up to London and he was proved right as Buemi picked up the points required.
The win overshadowed back-to-back wins for Buemi's teammate Nico Prost with Daniel Abt and Dragon Racing's Jerome D'Ambrosio taking the other podium spots.
There were further celebrations for Renault with Prost's win sealing a second successive constructors' title for the French team led by his father and four-time F1 world champion Alain.
All the talk, however, was about di Grassi and Buemi's controversial coming together.
"I was not expecting to celebrate it that way," Buemi said. "It was tough season. To start the race with what (di Grassi) did - i just thought he wanted to try anything.
Di Grassi, meanwhile, saw the incident differently.
"It's very clear from the data that Seb braked about 50 meters too early," di Grassi said.
"We tried our best. It was a real shame the accident -- we should have had a clean race. I'm proud of my season," the Brazilian added.
For Buemi, the win made up for the disappointment of losing out by a single point to Nelson Piquet Jr in last year's Formula E championship and more recent heartbreak in Le Mans last month.
The Swiss was part of the Toyota team who had dominated this year's annual 24-hour endurance race only for their hopes to be dashed by a mechanical failure on the final lap.
"I felt massively under pressure this weekend," he said. "To win is a big achievement."