The hosts controlled much of the contest but could not find the finishing touch until captain Kim Little fired home from the spot on 57 minutes after Niamh Charles was punished for her challenge on Caitlin Foord.
Steph Catley’s second-half effort from distance hit the crossbar and the Gunners, who were largely the dominant side, looked set to seal all three points, but the resilient Blues came alive in the final 10 minutes.
Jelena Cankovic was brought on for Sophie Ingle and quickly made the most of her 81st-minute substitution, sending a lovely long ball to Kerr who nodded in the equaliser on the stroke of full-time.
The result ensured the three-time defending champions remained three points clear of Manchester United, who moved into second place with a 6-0 thumping of Liverpool.
United are now level on 25 points with Arsenal, who dropped to third, but their prolific afternoon gave them the advantage on goal difference, netting a total 24 so far this season to the Gunners’ 20.
It took just six minutes for Lucia Garcia to fire in the opener before Alessia Russo and Hayley Ladd made it three before the halfway point.
Emma Koivisto’s 63rd minute own goal added to Liverpool’s woes, which were exacerbated when Martha Thomas and Rachel Williams put the game firmly out of reach.
Everton moved up to fifth position after a 3-2 victory over Reading.
Katja Snoeijs opened the scoring with a low shot from the edge of the area in the ninth minute, before Jess Park made it two for the Toffees.
Justine Vanhaevermaet pulled one back for the visitors on the hour but Gabrielle George netted for the hosts just a minute later, and Reading were only able to claw one more back through Deanna Cooper.