The Blues, reigning WSL champions, moved up to 49 points from 19 games, with United one point ahead having played one more match. The sides meet at a sold-out Wembley Stadium in the FA Cup final on Sunday.
Norway winger Guro Reiten opened the scoring in the eighth minute and Erin Cuthbert added a second before Danish striker Pernille Harder netted a brace to send Chelsea in 4-0 up at the break as they cantered to a 16th WSL victory of the season.
Arsenal also netted four first-half goals as they beat Brighton & Hove Albion 4-0. Again it was a good night for the Scandinavians, with Swede Stina Blackstenius netting twice and Norway's Frida Maanum notching a goal and an assist.
Arsenal are third on 44 points, Brighton are ninth on 16 and Leicester are 11th three points further back. The relegation spot in the 12-team league is occupied by Reading on 11 points.
