The Saints striker was the epitome of cool as he sent Artur Boruc the wrong way from the spot on 55 minutes, after Celtic skipper Stephen McManus had sent Craig Dargo tumbling in the box.
It was a win that was extremely unexpected given that Gus McPherson's men were on the wrong end of a 7-0 SPL drubbing last week but, everything that went wrong at Celtic Park seven days ago went right this time.
At times, for a quarter-final cup tie, the game seemed to have the feel of a league game and for St Mirren, anything less than conceding seven again was always likely to be an improvement.
They started the game well by nullifying any early Celtic threat, although it was the visitors that came closest in a stuffy first half.
Andreas Hinkel's delivery from the right was narrowly missed by Scott McDonald in the centre, but found Jan Vennegoor of Hesselink loitering at the back post.
The Dutchman's effort summed up his day when he got his footing wrong and only succeeded in scooping the ball into the side-netting.
Celtic huffed and puffed at the St Mirren defence, but didn't get much out of them, with Scott McDonald being ruled offside as he stuck away Shunsuke Nakamura's cross.
Will Haining then stood strong to deny the Celtic striker, who took one touch too many in the box, allowing the centre-half to swoop in and block the Aussie.
The penalty award was given when Dargo was released into the box by Dorman, but the Buddies hit-man was felled by McManus as he was about to pull the trigger, leaving referee Charlie Richmond no option but to point to the spot.
In the final stages, a hashed Mo Camara clearance for St Mirren and a second penalty shout for another foul on Dargo didn't alter things and the Saints faithful were allowed to enjoy a memorable victory and the prospect of a Scottish Cup semi-final.