Foley finished off a breakaway move that started deep inside the Waratahs own half four minutes from time to seal a home final next weekend against the Crusaders, who beat the Sharks 38-6 in the earlier semi-final.
The Waratahs, who lost both their previous finals to the Crusaders in 2005 and 2008, had been set on the path to victory by opportunistic tries from winger Alofa Alofa and centre Kurtley Beale at the start of each half.
The Brumbies, beaten finalists last year, played a full part in a hugely physical contest but were only able to cross once through winger Henry Speight in the 31st minute and failed to score after the break despite dominating possession.
More to follow...Scorers:
Waratahs: 26 - Tries: Alofa Alofa, Kurtley Beale, Bernard Foley; Conversions: Foley; Penalties: Bernard Foley (3).
Brumbies: 8 - Try: Henry Speight; Penalty: Christian Lealiifano.
Teams:
Waratahs
15 Israel Folau, 14 Alofa Alofa, 13 Adam Ashley-Cooper, 12 Kurtley Beale, 11 Rob Horne, 10 Bernard Foley, 9 Nick Phipps, 8 Wycliff Palu, 7 Michael Hooper (captain), 6 Stephen Hoiles, 5 Jacques Potgieter, 4 Kane Douglas, 3 Sekope Kepu, 2 Tatafu Polota-Nau, 1 Benn Robinson
Substitutes:16 Tola Latu, 17 Paddy Ryan, 18 Jeremy Tilse, 19 Will Skelton, 20 Mitchell Chapman, 21 Pat McCutcheon, 22 Brendan McKibbon, 23 Taqele Naiyaravoro
Brumbies
15 Jesse Mogg, 14 Henry Speight, 13 Tevita Kuridrani, 12 Christian Lealiifano, 11 Robbie Coleman, 10 Matt Toomua, 9 Nic White, 8 Ben Mowen (captain), 7 Jarrad Butler, 6-Scott Fardy, 5-Sam Carter, 4-Leon Power, 3-Ben Alexander, 2-Josh Mann-Rea, 1-Scott Sio
Substitutes: 16 Ruaidhri Murphy, 17 Ruan Smith, 18 Allan Alaalatoa, 19 Fotu Auelua, 20 Tom McVerry, 21 Michael Dowsett, 22 Joe Tomane, 23 Pat McCabe