Chilwell equalised in the 76th minute, to cancel out Michail Antonio's opener just after the hour mark, and then turned provider for Kai Havertz's winning goal two minutes from time.
The Blues still had to survive a late Hammers rally, with the visitors convinced they had levelled the game at 2-2 late on through Maxwel Cornet after Chelsea keeper Edouard Mendy had weakly palmed the ball away at the feet of Jarrod Bowen.
But Cornet's effort was chalked off after a VAR check ruled a questionable foul by Bowen on Mendy in the build up.
The second half action was a far cry from a dull opening 45 minutes in west London, where Chelsea defender Wesley Fofana (21) started just three days after his reported £70 million ($81.3 million) move from Leicester.
The match eventually sparked into life in the 62nd minute when Antonio scored from close range shortly after Mendy failed to punch clear from a West Ham corner.
Declan Rice collected the loose ball and squared to Antonio who only had to poke it across the line.
With Chelsea facing the prospect of a third defeat in six matches, Chilwell - sent on by Blues manager Thomas Tuchel - turned the game in the hosts' favour.
Thiago Silva's pass split the West Ham defence, and the substitute headed the ball on to himself and then spun clear before shooting through the legs of Hammers goalkeeper Lukasz Fabianski.
Cornet should have put West Ham back in front with his first touch off the bench, but he headed against the post from a Said Benrahma cross.
Chelsea immediately made the visitors pay for that miss, when Chilwell's low cross found Havertz, who tapped home to make it 2-1.
Moments later, Cornet seemed to have made amends for his earlier miss only for VAR to deny him a goal to the evident disgust of the away supporters.
