Then, at last, Chelsea sliced through the red wall of defenders. As Pernille Harder broke down the right, Kerr peeled away into space, anticipated the cross, and stabbed it past FIFA Best Women's Goalkeeper of the Year Mary Earps.
It was Kerr's fifth goal in three FA Cup finals, and her 10th in six domestic finals with Chelsea, celebrated with her trademark backflip somersault.
It also proved to be the only goal of Sunday's game. One week after bearing Australia's flag at the King's Coronation, Kerr had carried Chelsea to another crown.
This was a royal occasion too with Prince William among the 77,390 in attendance, a world record for a women's domestic club match. An Aston Villa fan, he must have been disappointed when Chelsea beat Villa in the semi-final (Kerr got the only goal, obviously), but hid it well.
The tie almost had a sensational start, Manchester United having the ball in the Chelsea net after 16 seconds through Leah Galton. But Ella Toone had run offside as she broke down the right before delivering the cross Galton converted.
However, the 'goal' settled United who were playing their first final, and in most cases making their fist appearance at Wembley.
They went on to control the first half while Chelsea struggled to provide Kerr with any service.
Chelsea keeper Ann-Katrin Berger had to claw away a deflected shot by Millie Turner, Nikita Parris was fouled fractions outside the box, and both Galton and Alessia Russo had chances.
Yet Chelsea would have led at the break but for Earps touching a 41st-minute Lauren James header onto the post.
In theory Kerr led the line with support from Canada's Jessie Fleming, 79 days before the pair will be in opposition in Melbourne at the Women's World Cup.
But United's midfield strangehold meant the combination never functioned and, shortly before the hour, Harder replaced Fleming.
Almost immediately, Kerr, released by another sub Sophie Ingle's first touch, brushed aside Maya Le Tissier and crossed for Harder but the Dane was unable to shoot quickly or powerfully enough.
Nine minutes later the roles were reversed. Harder's cross was perfect, so was Kerr's finish, and Chelsea were on their way to their third FA Cup triumph in succession.
United forced a few goalmouth scrambles as they searched for a leveller, but had no one with the finishing touch of Kerr, who was named player of the match.
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