Winger Henry Speight ran in two tries as Brumbies boosted their Super Rugby play-off ambitions with a 22-16 triumph that almost certainly ended the hopes of the Bulls.
Brumbies appeared on course for a comfortable win in Canberra on Friday when Speight's second try in the 58th minute left them 22-9 in front.
The visitors threatened a comeback when Pieter Labuschagne touched down from the back of a rolling maul late on, but Frans Ludeke's men made poor attacking decisions in the closing stages.
A victory for the Brumbies leaves them provisionally top of the Australia conference, although they will be replaced by Waratahs if they beat Lions on Saturday.
Defeat for Bulls, meanwhile, means other results need to go their way for them to extend their season.
Bulls captain Pierre Spies conceded his side had been the masters of their own downfall.
"It was a battle, you have to give credit to the Brumbies, but I think we made too many mistakes and we could have converted on those things, " Spies told reporters.
"We will have to look at that for next week's game against the Rebels in Melbourne. "
Crusaders kept their hopes of reaching the play-offs alive after defeating league leaders Hurricanes 35-18 in Nelson.
Mitchell Drummond, Nafi Tuitavake, Matt Todd and Nemani Nadolo all scored for the seven-time champions, but Crusaders - who are five points adrift of the wildcard places - remain at risk of missing out on the finals for the first time since 2001.
In Friday's other game, Rebels - who had prop Laurie Weeks red carded for retaliating to a Jannie du Plessis punch - slipped to a 25-21 defeat against Sharks, despite a converted 74th-minute try from Bryce Hegarty.