The Portuguese surged towards the Celta box before blasting a shot into the backof the net with a quality strike. Ronaldo completed his brace in the 58th minute as Los Blancos pulled away even further.
Kane has reached 20 league goals in 14 games, demolishing the old record set by Hamburg legend Uwe Seeler, who took 21 games to reach that mark in the Bundesliga's first season back in 1963-64.
Arsenal did have the ball in the net, Jakub Kiwior heading home after beating PSV keeper Walter Benitez to a free-kick, but it was ruled out for offside against Gabriel.
Toney thought he had broken the deadlock, volleying into the net from Mbeumo's cross in the 72nd minute, only for the effort to be disallowed for offside.
Few would have predicted such a thriller between two sides which have had their issues in front of goal this season but both had hit the net inside four minutes.
Chelsea, who were minutes away from a third successive away win, stay 10th, but head to Wembley for next weekend's Carabao Cup final on the backof one of their best displays of an erratic campaign.
A long ball from left-back Aleksandar Kolarov was headed on by Iheanacho into the path of Kevin De Bruyne, who took advantage of hesitation from Daley Blind to put the visitors in front.