With scores locked at 12-12 in Sunday's match at Ballymore Stadium in Brisbane, the Reds gave away an 80th-minute penalty for not rolling away.
Faitala Moleka stepped up to take the 22m penalty from straight in front, and the Brumbies fullback made no mistake with her kick just seconds after the final siren sounded.
Brumbies players celebrated wildly as the penalty sailed through, marking the club's first win since defeating the Reds 31-14 on March 30, 2024.
The victory snapped a seven-match winless run from the Brumbies, who had posted six losses and a draw since their last win over Queensland.
The home side were dealt a blow before the match when star fullback Lori Cramer was ruled out late due to a quad niggle.
Dominating at the start, the Brumbies had 89 per cent territory in the first 11 minutes.
But it was the Reds who struck first in the 19th, courtesy of a quick pick and go from prop Eva Karpani.
It marked Queensland's first foray into their attacking 22m, and the Brumbies made sure they hit back hard.
Just two minutes after Reds skipper Jemma Bemrose received a yellow card, the Brumbies unleashed a successful rolling maul to send hooker Tania Naden over.
The visitors made it two tries in five minutes when former Reds star Ivania Wong scored against her old side when she sped down the wing.
Reds outside centre Harmony Vatau levelled things up in the second half, but the ACT pushed hard for the win and gained their reward.
Wong was thrilled to achieve victory on her Brumbies debut and to score a try against her former side.Â
"It's super special to get the opportunity the Brumbies gave me and make my debut in front of family," Wong told Stan after the win.
"This was home for me for eight years, so it was a pretty special moment."
In the other round-one fixture, the NSW Waratahs kicked off their title defence with a 42-29 win over Fijian Drua on Saturday.
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