Sessegnon stooped to nod home a Sebastian Larsson free-kick with one minute left after Richard Dunne had appeared to have won it for Villa.
Dunne had risen highest to head the visitors back in front five minutes earlier after Connor Wickham had cancelled out Stilyan Petrov's stunning 20th-minute opener.
Bent, who was booed throughout on his first return to Wearside since his January move south, had missed a fine chance in the 70th minute when he shot straight at substitute keeper Kieran Westwood.
It had been clear from the pre-match warm-up that Bent was set for a torrid afternoon on Wearside after his controversial move to Villa in January.
And the striker was a virtual passenger in much of the early action with the main threat coming from Larsson, who fired a pair of dangerous free-kicks.
Bent got the chance he had been waiting for in the 16th minute when Emile Heskey's cross found the striker at the far post but he prodded wide under pressure from Keiran Richardson.
And Villa snatched the lead in stunning fashion in the 20th minute when Petrov was fed by Alan Hutton and lashed a stunning shot for only his sixth goal in 167 Villa appearances.
Sunderland came close to grabbing an equaliser within two minutes, when Bendtner found the head of Turner in front of goal but the defender headed inches over.
And they fashioned another chance in the 27th minute when Wickham headed goalwards in a crowded box but Stephen Warnock headed clear with keeper Shay Given stranded.
The home side snatched their equaliser in the 38th minute when Sessegnon turned Hutton and fed Wickham, who drove home a fine left-foot shot from 10 yards.
The Black Cats defence was still giving cause for concern and they had a mighty let-off in the 42nd minute when Petrov's free-kick trickled unchallenged across the face of goal.
Westwood got his first taste of Premier League action five minutes into the second half when Mignolet appeared to suffer a broken nose in the course of flapping away a Petrov corner.
The home side enjoyed their best spell of the game with James Collins twice forced to clear from dangerous positions, and Bendtner heading just over on the hour.
The home side had a penalty appeal turned down by referee Chris Foy in the 63rd minute when Bendtner met Larsson's cross from the right with a header which struck Hutton at close-range.
Larsson came close in the 70th minute after Chris Herd fouled Bendtner on the edge of the box and the midfielder's curling free-kick was well saved by Given.
And moments later Bent almost caught Sunderland out at the other end when he charged onto Herd's long ball before being pulled back for fouling Wes Brown as he wriggled clear.
As the game exploded into life, Westwood had his moment of glory when he got down to deny Bent from close-range after the striker had been put through by Gabriel Agbonlahor.
But Villa snatched back the lead in the 84th minute when Petrov lofted a free-kick from the left side of the box and Dunne climbed to head home.
Just when it seemed Villa had done enough to snatch the points, the home side roared back and Sessegnon headed home an 89th-minute equaliser from Larsson's free-kick.