Scotland’s Kim Little notched a brace after Vivianne Miedema’s opening goal and Katie McCabe and Leah Williamson were also on target as Jonas Eidevall’s side ran riot at Meadow Park to leapfrog rivals Tottenham on goal difference.
Fran Kirby opened the scoring inside two minutes with her 50th Super League goal, and Chelsea were three up at half-time.
Pernille Harder produced a brilliant solo run to double the lead before Sam Kerr tapped home a third.
Alessia Russo gave United hope with a deflected shot shortly after half-time but goals from Kerr, Drew Spence and Jessie Fleming made this emphatically Chelsea's day.
Having been stunned by Arsenal in their opening match, this was a performance to show that Emma Hayes' side are firmly back on track.
Tottenham maintained their perfect start with a 1-0 victory over Reading.
They had to wait until the 85th minute to make the breakthrough, Jessica Naz grabbing the goal.
Aston Villa won by the same scoreline at Brighton, with Emily Gielnik producing a fine finish three minutes after the break.
West Ham secured their first win of the season in fine style with a 4-0 victory over Leicester.
Australian debutant Tameka Yallop opened the scoring in the 26th minute, and Claudia Walker added her first goal for the club 12 minutes later.
An unfortunate own goal from Ashleigh Plumptre made it three just before half-time and Yallop netted her second late in the second half.
Additional reporting by the Press Association.