Turnbull came off the bench to make his first appearance since a Celtic medical unearthed a latent knee problem which required surgery and scuppered a £3million summer move.
But Ilkay Durmus stole the show with an 86th-minute winner to move St Mirren six points ahead of Ladbrokes Premiership bottom club Hearts and leave Motherwell without a win in eight matches.
Just like last week's eight-goal thriller between the teams in the William Hill Scottish Cup, which St Mirren edged on penalties, there was a marked difference in Motherwell's performance either side of half-time.
And they were made to pay as Jon Obika made it three goals in two games at Fir Park five minutes after the interval.
The hosts started with their new 3-5-2 formation and they made the early running.
Rolando Aarons snatched at a chance before winning the 11th-minute penalty after running from near halfway. Akin Famewo misjudged his tackle and although referee David Munro took a long look before pointing to the spot there were no complaints from the on-loan Norwich defender.
Donnelly had been denied by Vaclav Hladky in last week's penalty shoot-out but he gave the Saints keeper little chance as he found the corner of the net.
Other than a couple of decent long-range efforts from Durmus, the visitors offered nothing in the first half.
Motherwell forced 11 first-half corners, dominated possession and created a number of chances.
Hladky made a double save from Declan Gallagher's header and Tony Watt's follow-up and then denied the latter after the striker had run from his own half and created a shooting chance with some great skill.
Watt and Allan Campbell were off target following set-pieces, Hladky stopped Jermaine Hylton's shot with his feet and then scrambled to catch Aarons' free-kick as the on-loan Newcastle player threatened to score with a cross against the Czech for the second week running.
The visitors no doubt faced some stern half-time words from manager Jim Goodwin and the transformation was immediate. They took the game to Motherwell and Obika hit his 11th goal of the season from close range after Lee Hodson's driven cross ultimately landed at his feet.
The hosts settled down and Turnbull replaced Campbell with 19 minutes remaining but St Mirren soon created another good chance as Obika headed over under pressure from six yards.
Motherwell struggled to get the ball to Turnbull but the midfielder did set up a good chance for Watt, who could not get enough power on his header to trouble Hladky.
Watt soon hesitated when MacIver set him up and St Mirren broke before the ball fell for Durmus to lash high into the net from 16 yards.