Atdhe Nuhiu fired Sheffield Wednesday ahead from the penalty spot on 36 minutes at Griffin Park after James Tarkowski had been sent off for fouling him in the box.
Tarkowski, back in the Brentford side after a calf injury, bundled Nuhiu over as the Austrian striker shaped to shoot and referee Geoff Eltringham red-carded him after consulting his assistant.
Wednesday had been the better team, with Nuhiu heading Ross Wallace’s right-wing corner against the bar, having seen an earlier header saved by Brentford keeper David Button and another drift narrowly wide.
Wallace tormented Fulham at Hillsborough last weekend and his left-footed deliveries from the right flank have been a major problem for the Bees so far.
And Barry Bannan created a chance for Wednesday from the opposite side, sending in a superb cross towards the far post, where Marco Matias was just unable to apply the finish with a diving header.
The hosts eventually enjoyed a spell of pressure, with Marco Djuricin slicing wide from a corner by Alan Judge, who then crossed from the right for Jake Bidwell to volley over.
However, just as Brentford appeared to be growing in confidence, they found themselves a goal behind and down to 10 men.
Subs: Bonham, Woods, Yennaris, Barbet, Udumaga, Canos.