Premier League losses at home to Brighton and Crystal Palace led to intense scrutiny and pressure that had increased further after Tuesday’s tough Champions League defeat to Galatasaray.
Erik ten Hag’s men needed to go into the international break on a high but floundered for the most part on Saturday afternoon, with a string of errors resulting in Mathias Jensen’s opener.
But McTominay, brought on as a final roll of the dice in the 87th minute, had other ideas, firing home a 93rd-minute equaliser before heading home a scarcely believable winner four minutes later to spark wild Old Trafford celebrations.
It was an incredible end to a day that began in poignant fashion as Old Trafford paid tribute to Lady Cathy Ferguson following the death of Sir Alex Ferguson’s wife.
Players wore black armband and flags flew at half-mast on a day that both teams began with intent that they struggled to turn into clear-cut chances.
Aaron Hickey twice tried his luck from distance as the Bees attempted to sting a makeshift United backline.
With five defenders injured and the defence unbalanced, Victor Lindelof was deployed at left-back and Harry Maguire made his first league start of the campaign alongside Jonny Evans.
United lacked the Bees’ coherence and discipline and, shortly after mightily impressive Ethan Pinnock’s sliding block denied Mason Mount, their ongoing issues cost them.
Slack Casemiro gave away possession around the centre circle and failed to win it back, with Lindelof then failing to effectively deal with Yoane Wissa’s low ball into the box.
That botched clearance ricocheted off the Brentford forward into the path of Jensen to sweep home a low, first-time strike that Andre Onana could not get down to stop.
It was an all too familiar gut punch for United, who failed to muster a shot on target until Marcus Rashford wriggled free to test Premier League debutant Thomas Strakosha in the 37th minute.
Bryan Mbeumo sent a curling effort skipping just wide when Brentford returned to the attack, with United creating precious beyond a hopeful looping Casemiro header back across goal.
Boos greeted the half-time whistle and Ten Hag replaced underperforming Casemiro with former Brentford midfielder Christian Eriksen at the break.
The introduction’s hopeful, long-range piledriver forced Strakosha into action in the 53rd minute, with Rashford and Rasmus Hojlund trying to eke out an equaliser.
Diogo Dalot hacked clear after a long Brentford throw-in was flicked on, but play was now nearly entirely focused at the other end.
The right-back flashed an effort over and Hojlund struck into the side netting, with Strakosha palming away a Bruno Fernandes effort.
Lindelof went straight down the tunnel after being replaced by Anthony Martial, perhaps suggesting another injury for a United side struggling for luck or cohesion.
The France forward saw penalty appeals overlooked in between fellow introduction Alejandro Garnacho whipping over and Fernandes missing the target.
Onana impressively clawed away substitute Neal Maupay’s audacious 20-yard attempt but a few frustrated fans had begun heading towards the exit.
Anyone that left early missed a box office conclusion.
United thought they had equalised in the 89th minute, only to realise Martial had strayed offside when flicking a cross that deflected into the Brentford goal.
That setback made the stoppage-time turnaround all the sweeter for the Old Trafford faithful.
Three minutes into stoppage time Garnacho hooked back for Dalot to get a shot that Strakosha parried, with McTominay eventually controlling a clearance and drilling home.
Martial got a low shot on goal during a melee as United sought a winner that arrived in the seventh minute of stoppage time.
Fernandes’ floated free-kick from around the halfway line was headed on by Maguire and McTominay battled to head home to secure a jaw-dropping late victory.
