McTominay: Incredible Man Utd win my favourite moment
The Red Devils were starring at a third home defeat in eight days as they trailed to Mathias Jensen's first-half strike.
McTominay was only introduced as a substitute in the 87th minute, but brought his fine goalscoring form at international level with Scotland to the club scene just when United needed it most.
The midfielder had the ball in the net three times, but his first effort was ruled out for offside against Anthony Martial.
McTominay then pounced with a smart finish from a loose ball inside the area before heading in with virtually the last kick of the game.
"That would be one of my favourite moments on a football pitch. It was incredible," said McTominay.
"Everyone knows we're in a difficult situation at the minute but we've got fighters. The manager probably said 'go on and score' and thankfully I could repay that."
Despite a run of six defeats in nine games prior to Saturday's fightback, McTominay has had to be patient in getting his chance to shine under Erik ten Hag.
Victory lifts United up to ninth in the Premier League table and McTominay is hoping the dramatic turnaround can be the catalyst to turn around their season.
"Just never give up no matter the situation," he added.
"At this football club it is demanded that you never throw the towel in. For me I have been brought up here since I was five, I know that better than anyone."
Brentford boss Thomas Frank said defeat was "brutal" for his side to take after they seemed set to snap a seven-game winless streak.
"Football is brutal in moments like this. I think we did so many things right," said the Dane.
"Second-half you know you will be put under pressure here.
"I actually felt like we defended it very well, and didn't give any big chances away, well until the third minute of injury time."