Motherwell made it nine Premiership games without a win after losing to St Mirren in Paisley - and went joint bottom of the table in the process.
Former Fir Park forward Curtis Main bagged the only goal of the game after 16 minutes, as ex-Motherwell gaffer Stephen Robinson also got the better of his former employers.
In an incredible scrap at the bottom of the Premiership, four teams are now locked level at the bottom on 20 points with Kilmarnock, Ross County, Dundee United below the Steelmen, who are only ninth by virtue of a better goal difference and have a game in hand.
The defeat also means the Steelmen haven't won since a 1-0 victory away to fellow strugglers Dundee United on October 29, which was also the last time they kept a clean sheet in the league.
There was no debut for new signing Riku Danzaki as international clearance for the Japanese midfielder hadn't arrived prior to kick-off.
‘Well made two changes from last week’s Scottish Cup fourth round win at Arbroath, as Connor Shields and Kevin van Veen came in at the expense of new boys Ollly Crankshaw and Mikael Mandron. Worringly for the Steelmen, Mandron - who bagged both goals at Gayfield on his first start last week - missed out after picking up an injury in training on Friday.
Among the Saints starting 11 were a host of former Motherwell stars with goalkeeper Trevor Carson, captain Mark O'Hara, defender Charles Dunne and striker Curtis Main all formerly plying their trade at Fir Park, as did Declan Gallagher who started on the bench. Lewis Jamieson, who was recalled from his loan spell at Airdrie earlier in the week, was also among the Saints' substitutes.
The hosts had the first opportunity inside four minutes when the ball broke to Scott Tanser on the edge of the box and his venomous strike whistle past Liam Kelly's right-hand post.
Moments later a fluid move from Motherwell saw Shields break down the right, cross for van Veen to lay the ball off to McKinstry, but the winger's fierce strike was blocked at the vital moment by a Buddie body.
With 16 minutes on the clock, St Mirren broke the deadlock. A great bit of skill on the touchline from Alex Grieve played in Ryan Strain for a cross. His delivery across the face of goal was met at the front post by Main, who managed to pip his marker to the ball and poke it past Kelly from close range.
Despite the Steelmen seeing most of the ball, the Buddies continued to enjoy the best of the play and Ryan Strain had a good opportunity from a free-kick in 32 minutes after McKinstry brought down Dunne on the edge of the box. Strain's strike, however, was straight down the thrioat of Kelly and he gathered comfortably.
Half-time arrived with Motherwell yet to test Carson in the home goal.
Within 30 seconds of the second half, the Steelmen spurned a great opportunity to level. Blair Spittal whipped in a corner right on top of the Saints defence and although Ricki Lamie steered a powerful header towards goal, it flashed agonisingly wide of Carson's left-hand post.
In 56 minutes another chance went abegging for Motherwell when Spittal slipped in Shields down the right. His cutback fell perfectly for McKinstry, but instead of lashing it first time, he took a touch and cut back inside, allowing the Saints defence the chance to smother the ball and clear.
Steven Hammell turned to his bench for a creative spark with a double switch a minute later when Spittal and Shields were replaced by Ross Tierney and Olly Crankshaw.
In 72 minutes Crankshaw fizzed the ball across the box to the inrushing Max Johnston. The youngster made a superb connection with the ball to force Carson into his first real action of the game and he had to act fast to mop up the loose ball before van Veen tucked away the rebound.
The visitors kept pushing for an equaliser but Callum Slattery blzaing the ball high into the stands from 20 yards out in the final 10 minutes summed up the Steelmen's day in front of goal.
Seven minutes from time Main almost added his and Saints' second when his thumping strike on the angle flashed wide.
And in stoppage time McKinstry almost grabbed a point when the ball broke to him at the back post, but Carson made a tremendous stop with his outstretchd leg to deny him, amid a bit of a scramble in the box.
The Steelmen are back in action on Wednesday night when they host St Johnstone on league business.
ST MIRREN: Carson. Dunne, Fraser, Gogic, Tanser, O'Hara, Kiltie (Flynn 87), Erahaon, Strain (Gallagher 87), Grieve (Ayunga 60, Offord 78), Main.
Scorers: Main 16
Bookings: Grieve, Offord, Fraser
Subs not used: Urminsky, Kenny, R Taylor, F Taylor, Jamieson.
MOTHERWELL: Kelly, Blaney, Johnston, Lamie, McGinn, Spittal (Crankshaw 56), Goss (Cornelius 89), Slattery, McKinstry, van Veen, Shields (Tierney 56).
Scorers:
Bookings: Goss, Lamie
Subs not used: Oxborough, Dunachie, Johansen, O'Donnell, Ross.
HT: 1-0
Ref: John Beaton
Att: 6,424
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