St Mirren manager Jim Goodwin was frustrated to see the game suddenly go out of sight as two quick-fire second-half goals handed Celtic all three points at Parkhead.
Saints were one point off the bottom at kick-off but their defensive record was not far off the league leader's with nine goals conceded in 10 matches, only two more than the champions.
Their ability to close down space in and around their penalty box presented the hosts with a challenge and Celtic were restricted to long-range efforts in the opening quarter with Ryan Christie coming closest with a dipping effort.
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Saints were having their moments in possession, although their final ball normally let them down when they got into decent positions.
However, Jon Obika had a clear chance 25 minutes in when he brought down a long pass and held off Kristoffer Ajer. The striker got a left-footed shot away but it lacked power and Fraser Forster got down to save.
Celtic's biggest first-half threat appeared to be from corners and Christopher Jullien twice came close. The French defender headed over from six yards after beating Vaclav Hladky to Christie's delivery and saw another header cleared off the line by Ryan Flynn.
Christie appeared to be pulled back by Stephen McGinn as he prepared to meet a clever low delivery from another corner.
The visitors continued to crowd out Celtic in front of their goal but Forrest came close on the stroke of half-time when he got a yard of space and saw his shot touched onto the near post by Hladky.
With their game plan working to perfection in a stuffy first 45, it was a nightmare start to the second for the Buddies as Neil Lennon's side broke the deadlock just four minutes in.
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The opener came after Odsonne Edouard used his quick feet to bypass the crowd of pink and black shirts in front of him.
The Frenchman collected a pass from Scott Brown, shifted his feet and curled a shot off the inside of a post. Christie made a hash of the rebound but the ball ricocheted kindly for Mohamed Elyounoussi, who tapped home from close range.
Forrest then doubled the lead after good close control in the box, taking a pass and finding an angle to slot into the far corner from 14 yards.
With St Mirren having netted only five times in their first 10 league games, the quick-fire double made it mission impossible for Goodwin's side, but Celtic couldn't quite kick on, although they created several more openings.
Goodwin said: "The first-half was satisfying in terms of what we set out to do - be compact and not let Celtic play through us as they like to do, and try and hit them on the break," he said.
"I thought we did that to a certain extent, Vaclav (Hladky) wasn't as busy as maybe people would have expected, I don't remember many saves.
"We asked for a repeat performance and unfortunately we just didn't start the second-half in the manner we should have.
"The first goal was a bit unfortunate but the second was really disappointing because we could have defended it better on a number of occasions."
Celtic gaffer Lennon Lennon, meanwhile, hailed "priceless" winger Forrest after he celebrated his new contract with the second goal in the 2-0 win.
He said: "He was brilliant. We gave him a new deal and I don't care how much we pay him, he is priceless to me. That's 10 goals he has got.
"His inventiveness, intelligence was there for all to see. It was a brilliant goal and his all-round game was exceptionally high again."
With no Ladbrokes Premiership fixtures this weekend, St Mirren won't have a chance to bounce back until November 9 when they face Hearts at Tynecastle.
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