Leading through a Connor Goldson strike, Jamie McGrath scored either side of half time to put the home side ahead. Steven Davis found a leveller for Gers on 88 minutes, but in injury-time, Saints won the game with a Connor McCarthy scrambled winner.
The hours preceding kick-off saw a real deluge of rain fall here in Renfrewshire, but thankfully, the pitch here at the SMISA Stadium held-up well, and the rain finally went off .
That was to suit Gers early-on, and after beginning the match with almost total control of the ball, they hit the front through Connor Goldson.
Picking up midway inside the Saints’ half, Goldson drove forward, beat Etahn Erahon on the edge of the area, strode forward again and drilled a fine finish across Jak Alnwick and into the back of the net.
Gers continued to probe, and under pressure, Cedric Itten came close on the end of a James Tavernier cross, with Alnwick holding on, while Ryan Kent delivered a really teasing ball across the six-yard box which needed only a touch to take it into the net.
Conor McCarthy then made a great front-post block from another Kent cross to deny Kemar Roofe, and from the resulting corner, Jamie McGrath headed off the line to prevent Itten netting from the back-post.
Then, on 39 minutes, St Mirren were awarded a penalty, with Calvin Bassey bringing down Jamie McGrath in the area.
McGrath stepped-up to take the kick, and against the run of play, levelled matters by firing straight down the middle with Allan McGregor diving to his right.
Into the second half, and it was St Mirren who began the brighter before they took the lead on 52 minutes – McGrath the goalscorer again – taking in a Dylan Connolly cross before finishing beyond McGregor.
At the other end, Itten couldn’t get enough purchase on a Tavernier free-kick delivery, while James Tavernier thought he’d equalised from a Roofe cut-back – but the linesman adjudged Roofe to have carried the ball over the by-line.
Kent then had a shot held by Alnwick, while Tavernier saw an effort from outside the area end up with the same outcome.
Goldson headed a Tavernier corner wide before with two minutes of regulation time to play, Rangers dug out an equaliser.
From the very corner of the area, James Tavernier took a terrific free-kick which came back off the post but coming onto the loose ball to rifle into the roof of the net was Steven Davis.
It seemed the match was headed for extra-time, but St Mirren won the game in injury-time, Connor McCarthy eventually finishing after a scramble from a corner-kick.
RANGERS: McGregor, Tavernier, Goldson, Balogun (Hagi, 84), Bassey (Barisic, 56) Arfield (Davis, 56), Zungu (Kamara, 72), Aribo, Kent, Itten (Defoe, 72), Roofe.
SUBS: McLauglin, Helander, Patterson, Barker.
ST MIRREN: Alnwick, Tait, Mason, Shaughnessy, McCarthy, Doyle-Hayes, Obika (Erwin, 90), McGrath, Connolly (MacPherson, 82), Fraser, Erahon.
SUBS: Flynn, McAlllister, Durmus, Foley, Dennis, Lyness, Jamieson.
REFEREE: David Dickinson