The Bees substitute pounced during the closing moments of an extraordinary match which produced three red cards and four penalties to snatch a point for Uwe Rosler's side.
United looked set to record their second triumph under new manager Chris Morgan when Dave Kitson headed home following substitute Barry Robson's spot-kick.
But Brentford, who had taken the lead courtesy of Marcello Trotta's penalty, had other ideas and dragged themselves level in the 89th minute.
The visitors were creaking under the weight of some strong United pressure when Trotta fired them ahead from the spot following Clayton Donaldson's 24th-minute collision with United goalkeeper George Long.
Michael Doyle had twice tested Brentford's Simon Moore with powerful efforts while Kevin McDonald also went close.
Kitson, one of five players cautioned during a tetchy first-half, saw a low drive scrambled clear before heading Tony McMahon's corner wide before McDonald saw a volley fly over the Brentford crossbar.
Brentford skipper Tony Craig saw red after tangling with Kitson in the area but McMahon's spot-kick rebounded back off the crossbar.
Robson converted from the spot following Lee Hodson's handball before Harry Maguire followed Craig down the tunnel, with Sam Saunders missing a penalty.
Although Kitson's header looked to have settled the match in United's favour, Wright-Phillips pounced at the death after Donaldson had also been dismissed.