McKay returned match-winning figures of 5 for 28, including the final wicket of Lasith Malinga in the penultimate over, after earlier contributing a valuable 28 runs in Australia’s modest total of 231.
The Sri Lankans collapsed to 4 for 53 inside 10 overs in reply, before launching a trademark revival to set up yet another tense finish to end an entertaining tournament.
Upul Tharanga’s 71 and 60-run partnership with Lahiru Thirimanne (30) got the Sri Lankans back in the contest after McKay and Brett Lee (3 for 59) ripped through the top order.
But the visitors fell short in the end, bowled out for 215 with seven balls remaining.
“Clint McKay was absolutely brilliant tonight. The skill he showed, throughout the whole series but especially tonight, it was great to see him bowl so well,” Australian captain Shane Watson told Grandstand.
Australia claimed the best of three finals series 2-1, its first home tri-series victory since the 2005-2006 season and its first piece of tournament silverware since the 2009 Champions Trophy.
Sri Lankan opener Tillakaratne Dilshan was named player of the series.
Watson, skippering the side after Michael Clarke was ruled out with a hamstring injury, said the side was pumped up to deliver a bigger performance after failing to defend 271 in the second final on Tuesday.
“We knew we had to step up, especially after the last game, we were very disappointed,” he said.
“That’s a serious performance to defend 231 on that wicket, so I’m very proud of all the boys.”
McKay said it was fantastic for the bowlers to step up when it mattered.
“It’s been a bit of an indifferent series with the ball… it’s great tonight in such an important game, all the bowlers stood up and got us over the line.”
Sri Lankan captain Mahela Jayawardene said his side needed to build partnership at the start of its innings after bowling well.
“Australia deserved it, they played much better cricket than us today,” he said.
“I thought we managed to keep them 20-30 runs less, so it was a good total to chase.
“But we lost three of four wickets very quickly, and it was a tough task for the rest of the boys to try and creep through.”
Earlier, Australia’s solid batting platform built by top-scorer Matthew Wade (49) and his opening partner David Warner (48 from 45 balls) was eroded by a stunning collapse of 5 for 36 in a dozen overs.
But McKay smacked 28 runs from 32 balls, and combined with Lee (32 off 54) for a precious 40-run union for the eighth wicket.
Lee and McKay then united with the ball to tear through Sri Lanka’s vaunted top order.
A fired-up Lee snared the initial two victims – dangermen Tillakaratne Dilshan (8) and Kumar Sangakkara (19 off 9 balls).
Lee, potentially playing his final international match in Australia, marked both wickets with gusto, prompting a warning from umpires to tone down his celebrations.
McKay picked up the theme by dismissing Dinesh Chandimal (5) and captain Mahela Jayawardene (15) to leave the Sri Lankans wobbling.
The tourists were placed back on track by Thirimanne and Tharanga’s fifth-wicket stand, only for McKay to return and strike again to boast figures of 3 for 12 from his next six overs.
Tharanga was then united with Nuwan Kulasekara – the pair who almost pinched the first final in Brisbane with an audacious late-order partnership.
The duo cracked a rapid-fire 30 runs before Lee dismissed Kulasekara for 15.
Sri Lanka needed a run a ball from the final six overs when Tharanga’s match-high knock was ended when dismissed by Watson (2 for 13).
McKay, who expertly used changes of pace, then mopped up with the final two wickets to seal Sri Lanka’s fate.
“I haven’t got some of the pace of the other boys, so I’ve got to come up with a few different tricks and tonight it worked,” McKay added.
“It’s something I have been working hard on.”