Mensing had come close to hauling Saints level with a header in 56 minutes from a Darren Sheridan corner after Owen Coyle's shot had been palmed out by David McEwan.
His goal seven minutes from time was a thing of beauty as it hurtled through a packed defence before nestling in the bottom right-hand corner of the Accies net.
The result guarantees that Saints will end the season as Division One's runners-up and sets us up an intriguing battle for next term's top spot with new kids on the block and seemingly - even at this ridiculously early stage - title favourites, Gretna.
Everyone in Scottish football – apart from the hopelessly old romantics and those with a penchant for Hollywood-style 'happy endings' where the minnows eat the sharks – will be looking to Saints to putting a stop to the so-called 'wee club' Gretna's grand ambitions next season.
Coyle should have had a goal in the ninth minute but it was mysteriously chalked off for offside. He could never have been offside as the ball had bounced back out to him after a vicious Paul Sheering thunderbolt had cannoned off the post and out to the player-manager.
Coyle hadn't been offside when the ball left Sheerin's boot so the goal should have stood.