For the first 30 minutes, both sides battled the heat and entertained the crowd with good attacking play.
Ricky Gillies had a great chance to opening the scoring for the home side when he sped past James Sharp, but his attempted chip was weak and Darren Hill saved easily.
At the other end, Craig Hinchcliffe brought off a great save from a powerful long-range drive by Scott MacKenzie before holding a header from Mark Campbell.
St Mirren roared back and Kirk Broadfoot and veteran player-coach Mixu Paatelainen came close as the visitors defence was stretched repeatedly.
Not surprisingly, the pace dropped but David van Zanten thrilled the crowd with a good run which would have resulted in a goal but for Hill's brave block.
Falkirk's John O'Neil was sent off seven minutes after the break for spitting at Andy Millen and from then on Saints looked the likely winners.
Hugh Murray fired a fierce shot over the bar and Gillies curled an effort narrowly wide.
One aspect of St Mirren's play that lacked quality was their crossing, so it was strange that both goals came from excellent crosses. Murray sent a high ball from the left touchline and Paatelainen scored with a looping header, a trademark of the powerful Finn.
The clincher came nine minutes from the end. Laurie Ellis broke down the left wing and his measured cross was nodded down and beyond hill by Allan Russell from eight yards.
Hinchcliffe ensured a clean sheet with a breathtaking save from Kieran McAnespie in the closing seconds.