The England forward netted a superb treble on Sunday as Emma Hayes' confident side ran out 3-1 winners to move three points clear of rivals Manchester City and Arsenal in a closely-contested title race as the WSL season resumed following its winter break.
The victory was important for Chelsea after Arsenal edged out Everton on Saturday and City thrashed Liverpool 5-1 after another hat-trick from Khadija Shaw, with fourth-placed United now 10 points off the top.
Chelsea, without the services of Australia striker Sam Kerr for the rest of the season due to an anterior cruciate ligament injury, took the lead in the fifth minute when James fired past England team-mate Mary Earps.
Nathalie Bjorn, making her first WSL start for Chelsea since signing from Everton this month, produced a lovely ball over the top for James to double the hosts' advantage with a neat low finish in the 23rd minute.
Hayley Ladd pulled one back just before half-time to give Marc Skinner's visitors hope, but - to the delight of the majority of the 20,473 crowd - James made the points safe in the 85th minute when she broke clear one on one with Earps and made no mistake.
Additional reporting by the Press Association.