United have endured a torrid start to the season, including losing three of their six games at Old Trafford this season - having gone 31 games unbeaten at home before their loss to Brighton. Brentford themselves have struggled to get going this campaign, with the Bees winless in five league games.
It was Thomas Frank’s side that got off to the better start though, with early possession and chances falling to the away side, the best of which saw Aaron Hickey send a long-range effort within a whisker of the top corner.
It took the home side a while to make their mark on the game but once they did, they had all 10 outfield players camped inside Brentford’s half but struggled to unlock the watertight defence.
Just as United were beginning to become more threatening, they conceded due to individual mistakes - which has summed up their season so far.
Casemiro gave the ball away in his own half, allowing the Bees to pounce and break forward, and then a loose ball in the box was steered goalwards by Mathias Jensen, and Andre Onana could only get a weak hand to it - compounding his poor start as a United goalkeeper.
Marcus Rashford came close to equalising after drifting into the box from the left flank but his shot at the near post was gathered by Thomas Strakosha, ensuring Brentford went into half-time with their lead intact.
United have now trailed at half-time in four consecutive league games for the first time ever, so Erik ten Hag had a vital team talk with his players and they came out with urgency in the second half, pressing high up the pitch and were committing men forward but their struggle to break the Brentford defence down continued.
United began to turn the screw with 20 minutes remaining - first Bruno Fernandes tried his luck from range but his drilled effort was palmed away by Strakosha, and then Alejandro Garnacho saw a fizzing shot from inside the box fly over the bar.
Anthony Martial saw an effort ruled out for offside in the 90th minute, but the drama wasn't over as McTominay capitalised on a goalmouth scramble to fire home a last-gasp equaliser.
Brentford were stunned, and within four minutes of netting the leveller, McTominay had struck again to steal victory, heading past Strakosha to blow the roof off Old Trafford.
A win relieves some of the growing pressure on Ten Hag and fires United into the top half, while Brentford's winless streak in the league to a sixth match, as a heartbreaking conclusion denied them a first win away at United since 1937.
Flashscore Man of the Match: Scott McTominay (Manchester United)
