Chelsea are now in pole position in the standings on 37 points after 14 of the season's 22 league games, two points ahead of United and with a game in hand. Manchester City are third on 32 points.
United keeper Mary Earps (30) had to make two early saves from Kerr after Chelsea got in behind the high United backline, but the visitors' defence did not heed the warning and it was soon to be third time lucky for the Australian.
Lauren James (21) lofted a superbly-weighted long ball in behind the defence in the 23rd minute and Kerr chested it down expertly before lifting it over Earps for a sublime finish that takes the Australian to 22 goals in all competitions this season.
Alessia Russo (24) had a great chance to put United level 12 minutes later but her snap shot flew straight at Ann-Katrin Berger (32), and United also had a strong penalty appeal turned down when Nikita Parris (29) was brought down by Kadeisha Buchanan (27).
Despite their title hopes being in danger of suffering a serious blow, United lacked composure in the second half and they did little to threaten the Chelsea goal, rendering Kerr's strike enough for the Blues to secure a crucial win in the title race.
Later in the afternoon Liverpool full back Emma Koivisto (28) scored one goal and created another as her side came from behind to beat Tottenham Hotspur 2-1, while Everton were held to a scoreless draw by bottom side Leicester City and Aston Villa beat West Ham United 2-1.
