The Bluebirds substitute curled home a stunning free-kick as a dominant Owls side left Cardiff with just a single point following the 1-1 draw.
But Tomlin stopped Garry Monk's men from joining West Brom at the top of the Championship table when he curled in his fine set-piece with three minutes remaining.
Monk has been reaping the rewards of Harris's free transfer to Sheffield Wednesday from the Welsh capital.
The winger had scored two goals and provided an assist since joining Wednesday and was out to give his former employers another reminder of his talents on his return to Cardiff City Stadium.
Harris sent an early warning when he fired a half-volley over the crossbar, however, the Owls winger did not need a second invitation to snatch the initiative at his former home as he created the opener after 19 minutes.
Barry Bannan laid the ball off to Harris on the angle of the penalty area after his corner was only half-cleared.
Harris stepped on the accelerator to leave Marlon Pack chasing his shadow and sent a curling effort towards the far post.
The ball may have been heading for the back of the net, but as Neil Etheridge dived to his left, Borner stuck out a leg and diverted the shot away from the Cardiff goalkeeper and into the back of the net.
Steven Fletcher was next to test Etheridge moments later as the visitors piled on the pressure in search of a second, prompting Warnock to take drastic action.
Harris lined up the free-kick and smacked the angle of the post and crossbar with a curling effort that had Etheridge beaten.
Cardiff survived until the interval but were on the back-foot again after the restart. Some heroic defending from Aden Flint and Morrison ensured the Owls' lead stayed at a single goal, before the hosts finally started to threaten.
But the equaliser finally came after Tomlin dusted himself off after he was tripped by Massimo Luongo and sent a curling effort into the corner from 25 yards out to earn a point.