Lloyd Isgrove's early header put Southampton into a lead they thoroughly deserved at the interval.
But United levelled in the second half after Cody Cropper's mis-kick allowed Lawrence to strike into an unguarded goal.
Cropper's afternoon went from bad to worse when he was sent off for handling outside of the penalty area, and his replacement Will Britt couldn't keep out the resulting free-kick.
The visitors secured the points when Marnick Vermijl toe-poked in from two yards, but the result didn't tell the whole story.
On a sunny but cold afternoon at Staplewood, Southampton and Manchester United's Under-21s fought out a competitive contest with the hosts making the perfect start.
Omar Rowe burst down the left flank and supplied an inch-perfect cross for LLOYD ISGROVE to head home his second goal of the season.
Isgrove nearly doubled his tally after 20 minutes when his rasping left-footed drive was palmed away by the United shot-stopper Sam Johnstone.
Jack Stephens had a header flash across goal whilst United's most dangerous threat was Larnell Cole.
Jake Sinclair showed neat composure on the edge of the box, before firing a left-footed shot high and wide, as Saints went into half time the stronger.
HALF TIME: SOUTHAMPTON U21s 1-0 MANCHESTER UNITED U21s
The hosts began the second half brightly and could have doubled their lead had Omar Rowe's neat run along the by-line not been ended by Sam Johnstone.
Jesse Lingard engineered a good chance for United when he acrobatically volleyed past Cropper's left post.
Corby Moore had a long range effort flash wide before United scored their equaliser against the run of play.
It was a disastrous moment for Cody Cropper, who's poor clearance landed straight at the feet of TOM LAWRENCE. The Manchester United substitute was able to cut inside Turnbull and loft the ball high into the unguarded goal.
Lloyd Isgrove came close to restoring the hosts' advantage, but his daisy-cutter went inches past the upright.
17-year old Ryan Seager was brought off the bench to give Southampton more attacking options, but his afternoon was to be cut short.
When Ryan Tunnicliffe latched on to a long ball down the middle, Cody Cropper intervened by using his hand outside the penalty area. The referee immediately issued the red card, and Will Britt was brought on in place of the unfortunate Seager.
From the resulting free-kick ADNAN JANUZAJ put the visitors in front, and Southampton's prospects looked bleak.
The result was confirmed four minutes later, when another Januzaj free-kick was fumbled by Will Britt, and MARNICK VERMIJL tapped home from two yards out.
FULL TIME: SOUTHAMPTON U21s 1-3 MANCHESTER UNITED U21s
"I'm absolutely furious with the result" Southampton Under-21's coach Martin Hunter told www.saintsfc.co.uk
after the game.
"In the first half we played some great stuff, and [the players] carried out to a T what was expected of them.
"The first goal was a real good one to look at, and we've not really been troubled.
"We've been punished three times for our own errors, and that's the frustrating thing."
Southampton U21s: Cropper, Chambers, Martin, Turnbull, Targett, Moore, Curtis, Stephens, Isgrove (Seager 81 (Britt 84)), Sinclair, Rowe. Subs (not used): Young, Gape
Manchester United U21s: Johnstone, Vermijl, Veseli, Fornasier, McGinty, Brown, Cole (Blackett 90), Tunnicliffe, Lingard, Powell (Lawrence 46), Cofie (Januzaj 46). Subs (not used): Gollini, Ekangamene