With the score goalless in the 55th minute at Stamford Bridge on Saturday, the Blues' striking ace Kerr had just collected the ball inside her own half when Arsenal defender Catley raced up behind, clattered into her Australian teammate and robbed her of the ball.
Kerr clearly felt she had been fouled as she protested after being sent sprawling to the ground, but the referee allowed play on and Catley rampaged forward, playing a sharp pass to Alessia Russo, who in turn played in Beth Mead for a sharp match-turning finish.
Catley's mini-triumph over Kerr, the Australia teammate she'd replaced as national captain during Kerr's injury absence, proved a major turning point in the London derby between the English and European champions.
In the first half, Kerr had given her old friend the odd problem, not least when, at one point, she turned sharply in the box to wrong-foot Catley, who slipped and suffered a brief injury worry while the Chelsea striker whipped a cross-shot narrowly over.
But 32-year-old Kerr, who had begun to look a little weary after a fairly lively contribution to an evenly-poised match, was immediately taken off by Chelsea boss Sonia Bompastor after Mead's goal, leaving the Blues looking pretty blunt up front.
Arsenal, marshalled superbly at the back by Catley, simply grew in confidence with the excellent Mead laying on the second just after the hour-mark for their Spanish star Mariona Caldentey, who delivered a clinical finish.
It enabled Arsenal, who were also helped by a tireless afternoon's stint from Caitlin Foord, to celebrate their first away league victory at Chelsea since 2017.
More than that, it left Chelsea's hopes of yet another title in tatters after only their second league loss of the season, as they remain six points behind Manchester City, who will surge nine clear on Sunday should they win at London City Lionesses on Sunday.
