Forrest grabbed Celtic's second goal after the interval at Parkhead, just hours after signing a one-year contract extension which ties him to the club until 2023.
The 28-year-old came closest to breaking the first-half deadlock as St Mirren defended in numbers and displayed good skill and composure to net in the 54th minute.
Mohamed Elyounoussi had opened the scoring five minutes earlier from close range as Celtic ensured they remain top on goal difference.
"He was brilliant. We gave him a new deal and I don't care how much we pay him, he is priceless to me," Celtic boss Neil Lennon said of Forrest.
"He is in his peak years, it's 10 goals already only a quarter of the way through the season. That's a great return for a wide player".