The Belgium international turned away the Scot's goalbound 81st-minute header as interim boss Terry Connor's hopes of a first victory ended in disappointment once again.
Sylvan Ebanks-Blake had earlier passed up a glorious opportunity to put the visitors ahead at the Stadium of Light when he sliced wide to leave his side fighting desperately to keep their heads above water with time running out fast.
The visitors deserved something from an awful game played out in front of a subdued crowd of 37,476, but the point with which they emerged was less than they needed if they are to pull off a famous escape.
But they might have returned to the Black Country empty-handed had Sebastian Larsson not lifted an 89th-minute shot over Wayne Hennessey's bar from point-blank range.
Sunderland boss Martin O'Neill watched the reverse fixture from the directors' box at Molineux in December after being unveiled as Steve Bruce's successor, and a 2-1 defeat left him fearing for the club's top-flight existence.
Wolves went three points clear of the Black Cats that day, but the respective fortunes of the two clubs have changed dramatically since.
Before kick-off today, the rejuvenated Wearsiders had taken 31 points from a possible 57; in stark contract, the Midlands outfit had managed just eight, and found themselves nine points adrift of safety.
But with one side looking for a top-10 finish and the other intent on nothing other than survival, the first half unfolded as a largely dreary affair with both sides enjoying promising periods of possession without unduly troubling the other.
The best chance of the half fell to the lively James McClean with 16 minutes gone when Larsson and Sessegnon combined to play him into space on the left side of the penalty area.
However, with just keeper Wayne Hennessey to beat, the Republic of Ireland international shot tamely into the side-netting to let the visitors off the hook.
Larsson saw a curling 22nd-minute free-kick easily saved by Hennessey, who later gratefully plucked McClean's deflected effort out of the air after almost being wrong-footed.
In the meantime, Wolves had failed to punish Mignolet for spilling Anthony Forde's effort from distance and Michael Kightly dragged a shot wide from a promising position.
But as the half wore on, it was the Black Cats who started to dominate and full-back Phil Bardsley whistled a skidding drive just wide a minute before the break.
However, the visitors headed for the dressing room with their hopes of a first win for Connor still intact.
Sunderland returned in determined mood, but were all too often let down by their final ball as they failed to build upon a series of promising beginnings.
Sessegnon embarked upon a trademark high-octane run into the visitors' penalty area with five minutes of the half played, but his shot was well-blocked by defender Christophe Berra.
The home fans were growing increasingly impatient as their side failed to trouble Hennessey despite enjoying plenty of the ball, and their mood was not improved by the sight of Mignolet having to dive low to his left to keep out Forde's well-struck shot.
Mignolet might have been picking the ball out of his net two minutes later after Ebanks-Blake found space in the box, but drilled his effort wastefully wide.
Fletcher headed weakly at the Belgium international from Stephen Ward's 66th-minute cross with Wanderers enjoying their best spell of the game, but Nicklas Bendtner could do little better from McClean's ball at the other end three minutes later.
But the Irishman looked certain to put the home side ahead with 17 minutes remaining when he stopped to meet Craig Gardner's cross at the far post, but saw Hennessey make a fine block.
However, it was Mignolet who had to be at his best to keep out Fletcher's 81st-minute header to deny the visitors the three points they desperately needed.
Larsson capped a disappointing afternoon for the home side when he somehow turned Sessegnon's driven 89th-minute cross over the bar when it looked easier to score, and O'Neill's men might have paid the price seconds later.
Substitute David Edwards found time and space to send a dipping volley inches wide of Mignolet's right post, although the reprieve did not spare the home side a chorus of boos on the final whistle.