A 1-1 point was the least Brentford deserved after dominating throughout and hitting the woodwork on four occasions. However, it looked like Thomas Frank's men were to be hit by a sucker punch when Mount struck to ease the pain of his injury-plagued first season with the Red Devils.
But United failed to hold out, inflicting another blow to their chances of Champions League qualification, as Ivan Toney teed up Ajer to smash home.
Victories for Aston Villa and Tottenham earlier in the day upped the pressure on United to respond in the battle for Champions League football next season. A draw leaves Erik ten Hag's men 11 points off the top four and eight adrift of fifth-placed Tottenham.
But it was the manner of the performance that was just as concerning for Ten Hag on the back of a thrilling 4-3 FA Cup quarter-final win over Liverpool.