Mortlock's straight-running and composure in the backs and Smith's domination of the breakdown made the difference as the Brumbies yet again lowered the colours of their old foes from New South Wales.
With South African referee Mark Lawrence reverting to past habits at the breakdown by favouring the attacking side the Brumbies were able to put together their trademark continuity and ran out convincing winners in spite of losing skipper Stephen Hoiles because of a knee injury.
Waratahs flyhalf Kurtley Beale missed three penalties, twice hitting the uprights, to allow the Brumbies to go to the break 6-3 in front thanks to two penalties by Mark Gerrard and the home-side made the key incursion with a superb try five minutes after the re-start.
The touch-down went to Adam Ashley-Cooper but it started with Mortlock bursting strongly up the middle and was carried on with old-style Brumbies phase play - the count reaching 14 before the right-wing skidded in at the corner.
The Brumbies continued to maintain their momentum and replacement prop Ben Alexander had the honour of claiming the clinching try.
A clean break by inside centre Tyrone Smith split open the Tahs defences to allow the Brumbies to surge up to the goalline. The danger seemed to have been averted with a turnover but hooker Stephen Moore caught replacement scrumhalf Brett Sheehan napping and the upshot was the Brumbies forwards surging over the line with Alexander in control of the ball.
Gerrard's conversion took the Brumbies out to 18-6 with 16 minutes left to play and it was all over bar the shouting when the next points, a penalty by Gerrard in the 76th minute, also went to the home-side.
Lote Tuqiri's perfect kick-pass for Lachie Turner to run on to gave the Waratahs some consolation but there was too little time left to salvage a bonus point in what turned out to be the perfect result for the only two remaining unbeaten side, the Bulls and the Sharks.
Brumbies (6) 21: Tries by Adam Ashley-Cooper (44 min) and Ben Alexander (63 min). Mark Gerrard kicked a conversion and three penalties.
Waratahs (3) 11: Try by Lachie Turner (76 min). Kurtley Beale kicked two penalties.
