Sandy Baltimore's strike, in the fourth of five added minutes at the end of the first half, was the difference. Chelsea nullified a City side that had scored in each of its previous 21 games in all competitions and inflicted their first loss of the campaign.
The win moved Chelsea a point above City and opened the title race. Buurman was named player of the match.
Baltimore collected the ball just inside the box on the right from Ellie Carpenter, set herself and sent a piledriver into the roof of the net.
Chances at both ends had been limited until then. Bompastor said Chelsea also had a couple of great opportunities they did not take.
Bompastor is not usually one to make changes at the break. She still hooked Baltimore, who was likely still buzzing from the goal.
The sharper call came earlier. Bompastor withdrew Canadian defender Buchanan, and Chelsea players looked confused when the fourth official's board showed the number of the player Buurman would replace.
Asked to clarify, Bompastor said Buchanan had taken a knock to the head in training the day before, in contact with a team-mate, but had been fine."But for me, I was almost forced to do that because I know my player and I know she was not in the game,"she said.
"She had a really good performance against a top player,"Bompastor said.
City had their chances, but Chelsea stayed in control and defended resiliently to see out the narrow win.