The Force overcame four yellow cards to open their season with two victories for the first time in Super history following their home win over Moana Pasifika in round one.
They reclaimed the lead in the 75th minute when reserve hooker Brandon Paenga-Amosa burrowed over after 18 phases, and they managed to hold out the desperate home side for a famous victory.
Scrumhalf Nic White, who played 122 matches for the Brumbies across nine seasons before joining the Force last year, was at his feisty best.
He said he knew how hard it was for teams to win in his old home town.
"I'm just happy for the boys; we got away to a good start," White told Stan Sport.
"We knew coming down here, the Brums, they'll have their moments and when they came back we knew we were in a dogfight.
"Credit to boys, in those big moments, to be able to stick to our plan, stick to how we want to play.Â
"So, yeah, bloody proud - I know how hard it is to come here and get a win so that's pretty special."
It looked like it would be a match the visitors would remember for all the wrong reasons after letting slip an early 21-0 lead.
Hooker Nic Dolly, wingers Harry Potter and Dylan Pietsch and Carlo Tizzano all crossed in the first half, with the Force playing with intent and purpose.
The home side were bolstered by the inclusion of four Wallabies in Allan Alaalatoa, James Slipper, Tom Wright and Len Ikitau, who sat out their round-one win in Suva, but the Brumbies looked unsettled early on, making uncharacteristic mistakes.
There was still a healthy 28-14 buffer at halftime, but the Force's poor discipline started to undo their good work.
The Brumbies scored three tries either side of halftime, while Force No.8 Reed Prinsep was off the field for a clumsy tackle that also left flanker Tom Hooper ruled out of the match with concussion.
Winger Corey Toole crossed in the 53rd minute to give the home side the lead for the first time, but five minutes later flying Force recruit Pietsch was in for his second.
Brumbies No.8 Charlie Cale bagged his second for his team to again go ahead, then the Force lost both skipper Jeremy Williams and Nicholas Champion de Crespigny to yellow cards.
Playing with only 13 men for five minutes and trailing 42-38, it appeared the visitors would be denied.
But they showed their grit to hammer the home side's line, before Paenga-Amosa came up with the match-winner.
Brumbies captain Alaalatoa said he thought his team had worked their way to a win after their slow start.
"That was the chat out there, there was always belief that we could come back but if you think about that again, we can't start like that," the Test prop said.
"There's little areas off the ball which we weren't good at at the start and Western Force were good enough to capitalise on that so credit to them.
"You can tell that we were probably a little bit off the pace and we found our groove as the game went on but again, we just couldn't afford to start like that."
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