Brentford scored the only goal of what was a scrappy contest in the 12th minute.
Bristol City failed to clear from a Stewart Talbot free-kick on the left and Sam Sodje was allowed to latch on to a loose ball as the visitor's defence were caught dithering to stab home the winner form the edge of the six-yard box.
The home side continued to carve out the best chances of the first half. Both Scott Fitzgerald and Deon Burton could have done better from inside The Robins' penalty area.
The Bees almost extended their lead in unfortunate circumstances as Scott Brown's clearance cannoned off Jay Tabb and flashed narrowly wide with the keeper stranded.
After the break the game slowly started to turn in the visitor's favour and there were two desperate moments inside the Brentford penalty area during the closing 20 minutes of the match.
Firstly Stephen Brooker was put through on goal after Leroy Lita's flick on but his shot hit the keeper's shins and drifted away to safety.
And then in the 79th minute it was Brooker who again went close following a scramble in the Brentford penalty area but he could only stab an effort over the bar from close range.
As the game wore on Brentford continued to hold firm as Bristol City appeared to run out of ideas.
This fixture now puts Brentford back in the driving seat for a play-off spot, although boss Martin Allen says they will take it one game at a time.
He added that Bristol City played a 4-5-1 formation which: "made it hard to break them down. They didn't come at us, but we got the early goal to open up the game."
His opposite number, Brian Tinnion, said that his team lacked unity in the final third and added that: "We need to take 15 points from the next five games to stand any chance of making the play-offs."