Craig Sibbald's late volley kept his hometown team in the second tier next year after McKee had tapped in from a yard out - after missing from a similar distance earlier in the second half.
With Falkirk chasing the game to Myles Hippolyte's penalty, Alex Jakubiak laid the ball on a plate for McKee. At the backpost unmarked and with the goal wide open the former Morton mad contrived to swipe at fresh air, rather than knock in the vital strike from well inside the six-yard area.
Instead he had to wait 25 minutes longer for his strike that redeemed himself, and salvaged something from another lacklustre game from Falkirk. The tap-in would have been enough but Sibbald's late volley to seal the three points mean Saturday's opponents Dumbarton cannot catch Falkirk.
It was a bittersweet match. The Bairns missed a second half penalty when Craig Samson beat away Lewis Kidd's spot-kick but the important goals came minutes later and avoided a tension filled trip to Dumbarton for the second time in a year.
A dull procession of a first half turned into an urgent final half hour when ref Alan Muir penalised Paul Watson leaping in the box with Danny Mullen. The official blew for a pull by the Bairns man and pointed to the spot. To make matters worse Robbie Thomson went off injured just a few minutes later, with a thigh problem and was replaced by Conor Hazard.
Thomson had looked shaky and hesitant all night. With the ball at his feet he dawdled or booted the ball out of play. He was not alone though and in a first half with St Mirren cruising with the Championship title secure and relaxed smiles all round, Falkirk's slack passes and needless long balls characterised a poor display.
Gavin Reilly charged down one of the goalkeeper's clearances and the rebound flew over the bar, but it angered Paul Hartley. Much did in a poor first half with little of note. A point would have suited both teams and it showed.
Indeed a powderpuff first half became a powderkeg second as Falkirk's frustrations grew, so did the challenges. There was at least a bit of fight with the scores still blank until Alan Muir's decision confused the majority of the ground. Hippolyte gratefully accepted the invitation and beat Thomson from the spot, just two minutes after the keeper had denied Danny Mullen with an excellent stop on the line.
His goal infront of his former fans could have consigned his former employers to a relegation play-off, but the Bairns pulled through but put the away fans at St Mirren Park through a range of emotions.
Suddenly reality bit the Bairns and the ploughed forward. Alex Jakubiak should have teed up a perfect instant response and broke to the by-line but his cutback was missed by Joe McKee.
With the goal gaping, the ball trundling and completely unmarked, the midfielder somehow fluffed his lines and swiped at fresh air, missing the goal that should have levelled the game and all but ensured Falkirk's safety.
He did though with seven minutes to go when Kevin O'Hara headed a Sibbald cross off the underside of the bar and the simplest of finishes wasn't going to evade the midfielder a second time.
All that came after Lewis Kidd had a penalty beaten away by Craig Samson but more drama was to follow. McKee's leveller would have been enough to all but secure safety - barring a huge goal difference swing - but Sibbald swept in a late volley to keep the Bairns up in the SPFL Championship in the 90th minute.