It was Forest who almost struck first, as John Thompson was denied only by the bar, after keeper Stuart Nelson had only half cleared a fine looping cross from Nicky Southall, with Scott Dobie also seeing his attempt to prod home the rebound cleared off the line by Michael Turner.
But Brentford were far from overawed and, inspired by the intelligent running of the tricky Jay Tabb, they could have taken the lead themselves, as the winger's teasing low ball flashed across the box to Pratley, whose shot on the turn was superbly blocked by Thompson.
And it was Tabb who played an integral role as the visitors did fire the opening goal, after 23 minutes.
The Brentford man robbed Wes Morgan of possession as he lingered too long on the edge of his own box, and carried the ball to the byline, before crossing dangerously to the far post where Scott Fitzgerald saw his shot charged down, but Andrew Frampton was well placed, eight yards out, to fire home the loose ball.
Morgan made amends for his earlier error by tackling Tabb and kick-starting a Forest attack which culminated in David Johnson forcing a fantastic save from Nelson, as he connected well with Southall's looping cross.
Gary Holt became the second Forest player denied by the bar, with a dipping shot from 20 yards and Johnson headed into the sidenetting, as the home side found some confidence again before the half-time whistle.
And it took only two minutes, after the break, before Forest did find an equaliser. Nicky Eaden found room to cross from the right and pick out Thompson's run into the box, where he headed powerfully home from six yards.
From that point on, aside from a long-range effort from Tabb, that flew narrowly wide, it was Forest who were on top, with Johnson in particular looking lively.
The striker should have had a corner when his angled shot was deflected into the sidenetting and he was left frustrated again when Nelson produced a good reaction save, after he had battled his way into the box on the right side and engineered the room for a shot.
But, not for the first time this season, Forest were to pay the price for failing to make the most of their dominance as Pratley struck on the break to earn Brentford victory, as he glided onto the end of a pin-point pass from sub Ryan Peters, through the Forest defence, and rifled an unstoppable finish beyond Paul Gerrard.