This performance was a far cry from the impressive, swashbuckling Saints who wowed onlookers and confounded the doubters earlier in the campaign.
They toiled once again as a run of one win in five matches looked set to be extended, yet Mane’s late clipped effort saw Palace beaten 1-0 to halt their decline, moving Ronald Koeman’s men to within a point of the top-four.
For 83 minutes it had looked like Alan Pardew’s side would become the latest visiting side to frustrate the hosts at St Mary’s, with Eljero Elia managing the only notable effort despite Saints having as much as 75 per cent possession in the first half.
Martin Atkinson, refereeing his first match since the controversial 1-1 draw between Chelsea and Burnley, waved away a Dwight Gayle penalty appeal during that tepid opening period, although the Eagles improved markedly after the break.
Fraser Forster had to be alert to deny Jason Puncheon and Yannick Bolasie, with the latter save leading to Wilfried Zaha sending a low strike off the post as Saints continued to struggle at the other end.
It looked like being another one of those days until seven minutes from time, when substitute James Ward-Prowse burst through and struck an effort at Julian Speroni, whose weak parry allowed Mane to clip home to the clear relief of the majority here.
It was a welcome end to proceedings for Saints fans after a match which looked to be going the same way as so many recently, with Koeman’s men bossing possession without turning it into goals.
Filip Djuricic should have done better with an early close-range effort after a mis-hit Victor Wanyama effort fell into his path, while Damien Delaney had to be alert to cut out a low, fizzing cross from Mane.
The Senegal winger was proving a nuisance and Morgan Schneiderlin was impressing in the heart of midfield, hitting an audacious flick wide before trying his luck directly from a corner.
There was a sharp intake of breathe from those inside St Mary’s when Gayle went down on a rare Palace voyage forwards, although referee Atkinson did not deem Jose Fonte’s challenge on the edge of the 18-yard box a foul.
Zaha attempted to stab home after Forster spilled a cross as half-time approached, although it was Saints who were in the ascendancy.
Palace skipper Delaney did brilliantly to prevent Graziano Pelle getting on the end of a cross and Speroni denied Elia at his near post from the resulting corner.
Moments after the second half got under way, Forster was again required to react quickly, denying Puncheon after the former Saints midfielder wriggled free and got a shot away.
It was a warning shot Saints failed to heed as Bolasie forced the England goalkeeper into another save and, after the hosts failed with their clearance, Zaha saw a low right-footed strike come back off the far post.
The home faithful’s frustration was becoming audible as the familiar pattern of recent matches re-emerged.
Djuricic appealed for handball against Martin Kelly and then produced a moment of magic which saw a Clyne cross cut out.
Maya Yoshida almost directed home with his knee and Dusan Tadic curled a free-kick over as Saints continued to press, with Wanyama showing impressive skill to burst into the box before Joel Ward made a timely block.
Shane Long directed over after Kelly forced a save at the other end, before Saints finally made the breakthrough.
Ward-Prowse, brought on as a second-half substitute, ran through the middle and hit a low strike Speroni could only parry, with Mane reacting to coolly clip home.
It ended a desperately long wait for a Saints goal and proved the difference as, try as they might, Palace were unable to muster an equaliser.