Arsenal started the first half well, and looked to be shifting up through the gears, but a resolute Brentford defence proved difficult to unlock, and it was the Bees who perhaps should have gone in ahead at the break after Ivan Toney left the woodwork rattling.
Despite starting the second period at pace, Arsenal were unable to find a breakthrough until Mikel Arteta brought on Leandro Trossard, who had the ball in the back of the net at the back post within four minutes of entering the action.
But the lead did not last long, with Toney at last finding the target on 75 minutes, although VAR did have a long hard look at the build-up to find out if there was an offside in the build-up.