Just four days out from the opening Test with the Wallabies, the injury-hit Lions were uncharacteristically nervous and hesitant in the face of the Brumbies determination.
It was the first win by a provincial side against the Lions since Queensland beat the tourists in 1971 and they were also only the sixth Australian side to keep the Lions try-less.
The Brumbies, who have already qualified for the Super Rugby playoffs, were without six Wallabies in camp but they still had enough to embarrass the Lions before 21 655 fans in chilly Canberra.
They scored the only try of the match in the fifth minute through Tevita Kuridrani, while fullback Jesse Mogg converted three penalties.
The Lions points came from two penalties each from Stuart Hogg and his second half replacement Owen Farrell.
"We got physically beaten at the breakdown and set pieces and the Brumbies were hungrier than us," Lions skipper Rory Best said.
"We were a little nervy out there and we misfired at times."
The Lions encountered stiff resistance from the Brumbies from the kickoff and were rocked by some committed defence.
The Brumbies struck first when Fijian centre Kuridrani exposed some weak defence to score the opening try in the fifth minute.
Mogg initiated the chance with a long pass to Andrew Smith who linked with Kuridrani on the left-hand touchline.
Kuridrani easily beat the tackles from tour debutant Christian Wade and fullback Rob Kearney to score wide out.
The tourists resorted to high kicks from Hogg and Ben Youngs but they made little impression on the Brumbies, who looked the better side in the opening half.
The Brumbies missed three kicks at goal before Mogg landed a penalty for an 8-0 lead one minute from half-time.
The Lions faced going in scoreless at half-time for the first time since the 1980 tour of South Africa against Free State before Hogg kicked a penalty on the siren.
It was an uncertain opening for the Lions with several fumbles and two lineouts given away on poor throws from hooker Best.
Welsh wing Shane Williams, who was summoned to Australia to help the injury depleted squad, almost scored with his first touch before he was bundled into touch and on another occasion he was in the clear but was called back for putting his foot in touch.
Mogg stretched the Brumbies' lead to 11-3 with a 45th-minute penalty as the Lions looked clueless in attack.
Scottish prop Ryan Grant conceded a penalty for twisting the head of Scott Sio and Mogg made them pay with his third penalty and a 11-point lead.
Hogg pulled three points back with his 57th-minute penalty, but he hit the upright with his next attempt to howls of delight from the home crowd.
Coach Warren Gatland brough on replacements - Geoff Parling, Richard Hibbard, Alex Corbisiero, Dan Cole, and Dan Lydiate - to stir up the tourists and the introduction of fly-half Farrell got some much needed attacking rhythm.
Farrell kicked a penalty for the tourists to trail 14-9 with 16 minutes remaining, and the England number 10 pulled the tourists to within two points with another kick at goal with eight minutes left.
Toby Faletau was penalised for not releasing as the Lions went for the clinching try as the minutes ticked down.
But the Brumbies stoically held on to count down the clock to a famous win.
Teams:Brumbies
15 Jesse Mogg, 14 Henry Speight, 13 Tevita Kuridrani, 12 Andrew Smith, 11 Clyde Rathbone, 10 Matt Toomua, 9 Ian Prior, 8 Peter Kimlin (captain), 7 Colby Faingaa, 6 Scott Fardy, 5 Sam Carter, 4 Leon Power, 3 Scott Sio, 2 Siliva Siliva, 1 Ruan Smith
Substitutes: 16 Josh Mann-Rea, 17 Jean-Pierre Smith, 18 Chris Cocca, 19 Etienne Oosthuizen, 20 Jordan Smiler, 21 Mark Swanepoel, 22 Robbie Coleman, 23 Zack Holmes.
British and Irish Lions
15 Rob Kearney, 14 Christian Wade, 13 Brad Barritt, 12 Billy Twelvetrees, 11 Shane Williams, 10 Stuart Hogg, 9 Ben Youngs, 8 Toby Faletau, 7 Justin Tipuric, 6 Sean O'Brien, 5 Richie Gray, 4 Ian Evans, 3 Matt Stevens, 2 Rory Best (captain), 1 Ryan Grant
Substitutes: 16 Richard Hibbard, 17 Alex Corbisiero, 18 Dan Cole, 19 Geoff Parling, 20 Dan Lydiate, 21 Conor Murray, 22 Owen Farrell, 23 Simon Zebo