The SPL leaders continue to stumble towards the title after the former Hoops youngster cancelled out Kris Commons' early strike after Victor Wanyama had been dismissed
The Hoops had been comfortably ahead through a sixth-minute Kris Commons header, but a red card to Victor Wanyama and the spot kick saw their title charge continue to stumble.
Danny Lennon decided to stick with the starting line-up that beat Hearts at Hampden Park two weeks ago to claim the Scottish League Cup, their first trophy in 26 years, which meant new signing Sander Puri was forced to settle for a place on the bench.
The Parkhead side made two changes to the side that came back from two goals down to claim a dramatic last minute victory at home to Aberdeen, with Wanyama and Georgios Samaras returning in place of Mikael Lustig and Anthony Stokes.
Celtic came out with purpose early on and took the lead after only six minutes when the home failed to deal with a corner. Efe Ambrose gained possession on the edge of the area and chipped the ball in for Commons to head home from close range.
From there on, for the remainder of the first half, it was all St Mirren. Paul Dummett ran onto a ball from McGowan but his strike from the left wing was saved by Hoops goalkeeper Fraser Forster.
With less than a quarter of an hour gone the Paisley side had three chances to get on level terms, but Connor Newton, Marc McAusland and John McGinn all failed to convert.
The league leaders could have extended their lead with 10 minutes remaining in the half, as Beram Kayal cut the ball inside from the right to Commons and he tried his luck from the edge of the area, but the midfielder couldn't trouble Craig Samson in the home goal.
As the game entered injury time Neil Lennon was handed a blow as Commons suffered an ankle injury after Kayal fell on his team-mate after challenging for the ball with Esmael Goncalves.
With nine minutes gone in the second half Celtic found themselves reduced to 10 men as Wanyama was shown a straight red card by referee Bobby Madden after the Kenyan midfielder caught McGowan on the shin.
St Mirren could have drawn level midway through the second half when Gary Teale and David van Zanten played some neat football down the right before the latter arrived in the box, but his strike sailed into the stand behind the goal.
The hosts finally levelled with 10 minutes left as Goncalves went down on the edge of the area under a challenge from Emilio Izaguirre. Referee Madden pointed to the spot and McGowan fired home the equaliser.
They could have snatched all three points late but Teale's long-range effort sailed over Forster's bar and the spoils were shared.