The hosts edged the first half, with David De Gea being called into action on several occasions to keep the scores level.
But it was United who had the better of the second period, and goals from Anthony Elanga, Mason Greenwood and substitute Marcus Rashford sealed an important win for Ralf Rangnick’s side.
FIRST HALF - DE GEA ON TOP FORM
The hosts started the first half with great intensity, and almost took the lead two minutes in when Mathias Jensen dragged his attempt narrowly wide.
United responded to Brentford’s early pressure, with Elanga’s looping header causing confusion between Jonas Lossl and the Bees’ backline, before they were able to clear.