The 25-year-old turned James McClean's 51st-minute cross-shot past keeper Ali Al Habsi to win a match of little quality as 10-man Wigan were made to pay almost instantly for Jordi Gomez's rush of blood.
Gomez was sent off just two minutes into the second half for a dangerous lunge at defender Danny Rose and a game in which his side had enjoyed the better of the opening 45 minutes turned on its head.
Black Cats boss Martin O'Neill will have been delighted to see his team end a 12-game wait for a league victory dating back to last season, but it might have been very different had it not been for Gomez's lack of discipline and keeper Simon Mignolet.
The Belgium international pulled off vital first-half saves from James McCarthy and Arouna Kone to keep the visitors at bay as the home side struggled to find any real rhythm.
However, the sending-off gave them fresh impetus and they never looked like surrendering their lead.
The game represented a contest between a club with no Premier League win in a dozen attempts and another with just one this season, and the 37,742 spectators who attended perhaps should not have expected a thriller.
Sunderland were boosted by the return of England winger Adam Johnson, who had passed a late fitness test on his thigh injury, but deprived of the services of skipper Lee Cattermole following his midweek dismissal.
Cattermole's absence proved key before the break as Sebastian Larsson and Jack Colback struggled to get to grips with McCarthy and Shaun Maloney, who combined well with Kone and Gomez to stretch the home defence repeatedly.
Indeed, the visitors could have been ahead with just four minutes gone when Republic of Ireland international McCarthy was allowed to waltz through a statuesque Black Cats defence, only to see his left-foot shot blocked by Mignolet.
From the resulting corner, the ball was cleared only as far as Gomez on the edge of the box, and he blasted a volley over.
Mignolet came to the rescue once again with 15 minutes on the clock when Jean Beausejour crossed from the left to pick out Ivorian at the far post.
He seemed certain to score, but Mignolet somehow managed to bundle his effort from point-blank range around the post.
For their part, Sunderland created little of note with in-form striker Fletcher feeding off scraps for long periods with wide-men Johnson and McClean unable to provide him with any meaningful supply.
However, they came close to taking the lead with 28 minutes played when Sebastian Larsson's free-kick from the left evaded attackers and defenders alike as it sped across the penalty area before being hacked off the line by Ivan Ramis.
But the half finished as it had started with Wigan threatening once again two minutes before the whistle when McCarthy once again made the most of the time and space he was afforded to send a long-range effort inches wide with Mignolet diving desperately to his left.
Roberto Martinez will no doubt have been happy as his side returned to the dressing room, but his mood will have changed markedly within two minutes of the restart.
There seemed to be little danger as left-back Danny Rose picked up possession on halfway, but Gomez leapt into an ugly challenge to leave the Sunderland man in a heap and referee Howard Webb with little option to produce a red card.
The home side made their numerical advantage count with four minutes when McClean broke into the box and appeared to drag his shot across the face of goal.
Whatever his intention, the ball fell perfectly for Fletcher, who swept it home without a second thought to break the deadlock.
O'Neill's men were suddenly re-energised and immediately set about the task of killing the game off with Stephane Sessegnon forcing Ali Al Habsi into a 54th-minute save before Craig Gardner blasted over from long range.
Martinez sent on striker Franco di Santo for Maloney on the hour, but the game was rapidly slipping from his side's grasp.
Neither side was able to exert any measure of control as time ran down, but Sunderland held out in what was an untidy conclusion to claim three valuable points.