McManaman struck with his fourth goal of the season – and his second in the last four games – after he caught Birmingham napping in only the third minute at St Andrew’s.
It was just the start that Wigan were seeking as they had been limping towards the play-offs after losing their previous two matches against Burnley and Blackpool.
But for Birmingham, only three years after winning the league cup when they beat Arsenal at Wembley, they are now staring into the relegation abyss following an 18th successive Championship home game without a win stretching back to October.
Their fate is now out of their own hands as even a win at Bolton in the final game of the season on Saturday might not be enough to save them if other results go against them.
Wigan made the dream start that they had been hoping for when Jordi Gomez simply waltzed through the Birmingham defence.
He was allowed time and space to surge forward before playing the ball to his left to pick out McManaman.
The winger took a couple of paces forward before curling his low right-footed shot beyond the outstretched hand of goalkeeper Darren Randolph.
Considering the woeful run in front of their own fans, Birmingham did well not to buckle completely after the shock of falling behind so soon after kick-off.
Lee Clark’s side gradually got a foot-hold in the game and could have drawn level after 29 minutes only to be denied by a stunning save from former England goalkeeper Scott Carson.
After Birmingham skipper Chris Burke had seen his volley blocked, the ball fell kindly to Emyr Huws on the right hand edge of the area. His curling shot looked destined to fly into the top corner of Carson’s net only for the ex-West Bromwich Albion keeper to somehow palm the ball away for a corner.
Clark tried to nick a goal by sending on giant striker Nikola Zigic at half-time but he struggled to find a way past Wigan’s impressive central defensive pairing of captain Emmerson Boyce and Gary Caldwell.
When he did find an opening in the 69th minute, meeting a cross from right-back Paul Caddis, his weak header from six yards was comfortably dealt with by Carson.
Carson was by the busier of the two keepers and he came to Wigan’s rescue twice in a frantic finale from Birmingham – first diving bravely at the feet of Federico Macheda to deflect his shot away for a corner, before diving low to his right to cling on to a drive from Hayden Mullins.