A Freddy Eastwood strike was sandwiched between two goals from Iain Hume as Leicester looked set to take all three points back to the Midlands.
But Shrimpers midfielder McCormack grabbed a late goal that leaves them three points from safety.
United should have taken an eighth-minute lead when skipper Kevin Maher's throughball sliced open the City defence and found top scorer Eastwood.
His first shot forced a one-handed parry from goalkeeper Paul Henderson, who then saved the striker's follow-up effort.
But a minute later the lowly Blues were a goal down after Jason Jarrett picked out Levi Porter and the lively left winger whipped in a delightful cross.
Hume rose unmarked six yards out and headed home his tenth goal of the season.
Porter then turned a shot narrowly wide before setting up on-loan forward Geoff Horsfield with another sublime centre.
The Sheffield United striker looked favourite to score at the far post but his effort was turned out for a corner by Shrimpers goalkeeper Darryl Flahavan.
The hosts recovered to enjoy spells of possession but produced little end product to threaten Henderson.
And bang on the stroke of half-time Hume squandered a great chance to complete his brace when he was released by Alan Maybury. With just an advancing Flahavan to beat, the Canadian forward's angled shot went agonisingly wide.
Southend punished that mistake eight minutes into the second half when Eastwood grabbed his 16th goal of the season.
Maher picked out the striker on the left of the box, and when Gareth McAuley slipped, Eastwood turned and threaded the ball between him and fellow defender Richard Stearman low into the bottom of Henderson's goal.
Leicester responded with Hume blazing over from close range and then firing straight at Flahavan.
The Foxes' cause was handed a boost when Southend were reduced to ten men after left winger Richie Foran was sent off for a second yellow card when he brought down Jarrett.
And City capitalised on their numerical advantage in the 81st minute when substitute Hammond crossed from the right and Hume bundled the ball home as Blues defender Peter Clarke desperately tried to hack the ball off the line.
But Leicester were reduced to ten men themselves when Maybury was stretchered off injured following a tackle.
With numerical parity restored, Eastwood's shot was parried by Henderson but the loose ball fell to substitute midfielder McCormack, who gleefully lashed the ball into the empty net.