Having not beaten their opponents at GIO Stadium for eight consecutive games, the Waratahs snapped their losing streak on Friday in the Super Rugby Pacific.
But it wasn't pretty, even after the Waratahs' sparkling start resulted in a commanding 21-7 lead in the first half.
The Brumbies surged back to equalise in the second half, and the visitors looked on track to blow another apparent match-winning lead after being up 20-8 last Saturday against the Blues before going down 20-35.
But Canberra conceded three crucial penalties that led to goals for 20-year-old Sid Harvey in the final 15 minutes.
"For a young player (Harvey) who is four games into his career. It says a lot about where he is," former Brumbies coach and now NSW mentor McKellar said after ending a three-game competition losing streak.
"I just spoke to the boys about how we're a pretty new group together, and that we want to create special memories.
"Tonight's a special memory. To come into a place that we've got enormous respect for ... we fronted up and delivered."
Sent in to replace fellow Wallaby Harry Potter (quad) in just his second appearance of the season, winger Andrew Kellaway showed McKellar he should retain the starting No.14 jersey, scoring a try and running for 39 metres.
Unlike last Friday night, when the Canberrans overcame a 17-point deficit to beat last year's grand finalists, the Chiefs 33-24, the Brumbies couldn't get their offence to click late in the game.
Inspirational captain Ryan Lonergan was busy, posting 97 passes and 15 kicks in his side's last fixture before their bye.
Coach Stephen Larkham blamed himself for the loss.
"We'll question the program on whether we pushed them too hard this week," he said.
"It wasn't one of our better weeks (of training). Some of that might have transferred into the game. We should have corrected it in training."
The visitors started lively and were rewarded with three of the first four tries through Kellaway, Clem Halaholo, and Joey Walton.
The Brumbies closed the gap to 21-14 at halftime after Charlie Cale added another try to his competition-leading tally (nine) after the siren.
Second-rower Lachie Shaw scored twice, first in the 11th minute and then in the second half after receiving a spearing pass from Lonergan.
After Harvey's flurry of penalty goals, interchange Luke Reimer set up a thrilling finish when he crossed in the 78th minute, but the Waratahs hung on to secure victory.
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