Head coach Emma Hayes' side were just a point ahead of second-place Arsenal entering the final game of the season, and Chelsea's title hopes were in doubt as United's Martha Thomas and Ella Toone twice put the visitors ahead.
Kerr struck again on 65 minutes with another eye-catching goal as she controlled a looped header on her chest from 25-yards out before volleying over out-of-position opposing goalkeeper Mary Earps.
The achievement is Chelsea's third consecutive WSL title, and their fourth in the last five years, adding to their domestic treble last season.
Thomas gave Manchester United the lead with a deft header from Katie Zelem's free kick, and although Erin Cuthbert levelled four minutes later, Chelsea went in 2-1 down at the break after Toone's deflected strike for the visitors.
The Blues bounced back in the second half, with Australian Kerr levelling again in the first minute with a thunderous left-footed volley, and Norwegian winger Reiten put them in front with a deft finish five minutes later.
Kerr put the game beyond reach with a brilliant finish in the 66th minute, chesting the ball down and twisting to volley home after a poor clearance from Earps.
Arsenal had needed Chelsea to drop points to have any chance of snatching the title, and their win over West Ham was in vain.
Manchester City thumped Reading 4-0 to secure the third Champions League spot at the expense of Manchester United, who finished fourth.
Birmingham's 11-year spell as a top-flight club ended in midweek when a 6-0 thrashing by Manchester City condemned them to the drop, and they signed off with a 1-0 win over Aston Villa.
Information from Reuters was included in this report.