The Scot opened the scoring late in the first period, after Stephane Sessegnon's sliced effort fell right at his feet, and Saints were left without any answers in the second
In what was a huge game at the lower end of the table, both teams took a while to get going, aside from a splendid early effort from Stephane Sessegnon. The visitors took the lead through a fortunate Fletcher goal just before the break, and held on after it to secure three huge points.
Nigel Adkins made two changes from their last league match, Gaston Ramirez and Emmanuel Mayuka replacing injured Adam Lallana and dropped Jay Rodriguez. Martin O'Neill made just the one change, Titus Bramble making way for Danny Rose with Steven Fletcher fit to start.
Sessegnon nearly opened the scoring with just seconds gone in the game, but his powerful long-range effort was tipped expertly wide by Kelvin Davis.
The action was a lot less frequent thereafter, though. Jason Puncheon wasted a decent Saints opening when he missed the ball completely, before Rickie Lambert's free-kick was deflected just wide. Meanwhile, Sebastian Larsson forced a much more straight forward save from Davis, after neat work with Adam Johnson.
With just over half of the first period elapsed, Ramirez began to pull some strings in the hosts' midfield. The Uruguayan firstly forced Simon Mignolet to parry away hastily, before firing just over with another vicious left-footed effort.
It was O'Neill's side that opened the scoring just before the half-time break, though. Johnson delivered a dangerous low cross from the left, which intended target Sessegnon sliced horribly. However, fortunately for the visitors, it fell to Fletcher's feet and the Scot had no trouble tucking it away.
The second half started with a decent chance - Lambert shooting just over for Saints - however it was, again, uninspiring from both teams. Half chances saw Johnson blaze over for the visors and Jose Fonte's header deflect wide for Adkins men, but there was little cutting edge.
Things got scrappy as the half proceeded, Johnson being forced off after receiving a tough tackle and other small fouls breaking up play, with both sides' forwards feeding on scraps, too.
Sunderland, however, didn't mind one bit and held on to a crucial victory that pushes them further away from the drop-zone, while condemning Saints to the opposite, despite late chances for Ramirez and Guly do Prado going close and keeper Davis venturing forward for one last corner.