An early goal from former Bee George Saville proved the difference as Brentford fell to defeat at The Den.
Despite going on to dominate possession, the visitors paid the price for being caught cold in the opening minute.
Saville capitalised after Andreas Bjelland lost possession – and subsequently limped off – firing into the bottom right-hand corner.
Brentford looked to have equalised but Yoann Barbet was ruled offside when he nodded in after a corner on the left, and John Egan later headed against the bar.
Brentford (4-1-4-1): Bentley; Barbet, Bjelland (sub Mepham 3), Egan, Dalsgaard; Woods; Sawyers, Canos (sub Judge 74), Yennaris, Marcondes (sub Maupay 58), Watkins
Subs not used: Daniels, McEachran, Mokotjo, MacLeod