The Bees punished Oldham for a rare defensive lapse to secure victory at Boundary Park despite being reduced to ten men in the 57th minute.
Another substitute, DJ Campbell, got his marching orders after he jumped for a header with big centre-half Guy Branston and was adjudged to have used an elbow.
The straight red card for violent conduct will earn Campbell a three-match ban, but his side were ruthless at the back and held on to the single goal hit by Fitzgerald just nine minutes later.
Fitzgerald produced the right response after recently being put on the transfer list by Brentford manager Martin Allen.
Allen's team have now claimed back-to-back wins following a dip in form but Oldham have lost two on the bounce and are sliding down the table.
It was the Latics who played the better football in the first half and made chances that were wasted by Chris Killen and Richie Wellens.
They almost went ahead on 36 minutes but Mark Hughes, making his first league start of the season, headed against the bar from six yards.
Oldham were rescued by goalkeeper Chris Day five minutes before half-time after Brentford were awarded a controversial penalty by referee Tony Bates.
Bates saw Branston's challenge on Lloyd Owusu as being just inside the box. Oldham vented their fury but Day kept them level with a fine save to his left from Kevin O'Connor's kick.
Brentford's defence got meaner as the game went on and visiting goalkeeper Stuart Nelson only had to make routine saves from Killen and midfield veteran David Eyres.
Campbell, who had gone on for the injured Paul Brooker in the first half, was sent off early in the second period but the visitors were not to be denied.
Owusu charged down the right and crossed to the near post, where Fitzgerald controlled the ball before calmly steering a shot inside the far post from eight yards.
Darren Pratley had a chance to double the lead, but Oldham could only manage a shot from Paul Warne that flew over the bar as they chased the game.