The Bees' substitute coolly side-footed home from 10 yards with his first touch of the game after coming on as a replacement with 20 minutes left.
The scoreline would have been far more emphatic but for a trio of smart saves from Scunthorpe goalkeeper Sam Slocombe, who denied Bees striker Marcello Trotta.
Slocombe, who could easily have seen red when he handled outside the box to deny Bradley Wright Phillips early on, was quick to thwart Italian Trotta with reflex saves either side of the break.
The visitors should have taken the lead midway through the first half, but Tom Newey blazed over from eight yards after a flowing three-man move.
Wright-Phillips forced a full-length stop from Slocombe just after the break and skipper Jonathan Douglas's lob lacked the power to trouble the Scunthorpe keeper.
The breakthrough forced Scunthorpe to abandon their defensive set-up and go in search of an equaliser and Iron winger Mark Duffy flashed a free-kick just over late on.
Brentford's bid to grab the second almost came off in the last 10 minutes, but Wright-Phillips sliced wide and substitute Stuart Dallas's dipping shot was tipped over.