Caroline Weir's 48th-minute strike sealed a historic derby victory for Manchester City in front of a record Women's Super League crowd of 31,213 at the Etihad Stadium.
Weir's left-foot shot from outside the box proved enough to take the points but only after Manchester United, who dominated the first half for long periods, came agonisingly close to snatching a draw.
City goalkeeper Ellie Roebuck saved brilliantly to deny Jane Ross before the break, then was fortunate to see a late Jackie Groenen effort rebound back off the base of a post and into her arms.
Featuring in the top-flight for the first time after cruising to the Women's Championship title last season, Casey Stoney's side did enough to suggest they can be a significant force this season.
Having survived an early scare when keeper Mary Earps struggled to clear a loose back-pass, the visitors – cheered on by a sizeable contingent in a crowd which dwarfed the previous WSL record of 5,265 at Brighton last season – rose to the challenge.
Only a superb, twisting save from Roebuck prevented Ross from opening the scoring for United in the 22nd minute when she got on the end of a left-wing cross from Leah Galton.
City defender Gemma Bonner made a vital interception to deny Ross another shooting chance, then Ross and Groenen just failed to squeeze the ball over the line from another Galton cross.
Nick Cushing's City side, intent on wresting the WSL title back from Arsenal, looked sluggish and short on ideas until late in the first period, when Janine Beckie and Weir both carved out half-chances.
But it was a different story after the break as City began to dominate and they grabbed the lead within three minutes of the restart when Weir let fly with a rising shot from outside the box.
Beckie fired over the bar then missed a further opportunity after racing over half the length of the pitch in a lightning counter-attack, only to miscue her final effort from the edge of the area.
Stoney's side seemed to visibly wilt as City completely dominated the final half hour, pinning the visitors back in their own half.
City almost doubled their advantage in the 81st minute when the impressive Beckie played a neat lay-off in the box and Tessa Wullaert screwed a rising shot just over the bar from a tight angle.
But there was still time for United to come agonisingly close to snatching a point when Groenen was found unmarked in the City box in the 83rd minute.
Despite stumbling as she attempted her initial shot, the Dutch striker trickled an effort which beat Roebuck but rebounded off the foot of a post and straight back into the relieved keeper's arms.
:: Victoria Williams missed a penalty for the visitors as the Women's Super League match between Bristol City and Brighton at Ashton Gate ended goalless.
