The Black Caps needed only 21 overs to surpass Sri Lanka's total of 188, having set up their victory by taking five wickets inside 10 overs despite the tourists winning the toss and electing to bat on a good pitch.
A sell-out crowd attended Hagley Oval, with fans eager to see one of the final appearances of McCullum, who announced heading into the game that he will retire from international cricket in February.
He treated the fans to a trademark ballistic innings by scoring a half century from 23 balls as he and opening partner Martin Guptill put on 108 runs in just 10.1 overs.
McCullum made 55, Guptill 79 and Henry Nicholls scored 23 not out on debut, hitting the winning runs.
The opening pair scored at more than eight runs per over through the first five overs then accelerated and raised New Zealand's 100 from only 9.1 overs to snuff out any hope Sri Lanka may held that their total was defensible.
"It was probably a 280, 290 par wicket so it was a matter of not getting to far in front of ourselves, trying to remain controlled although it probably didn't look like that from a distance," McCullum said.
"Once we got a partnership underway we knew it was going to be tough for the bowlers on a wicket on which there was just no margin for error."
Sri Lanka captain Angelo Mathews had been delighted to bat first, describing the pitch as a "belter" for batsmen. But he saw his top order disintegrate in the face of accurate fast bowling from Matt Henry and Mathews himself was out for a first ball duck as Sri Lanka crashed to 27-5 inside 10 overs.
New Zealand did not miss pace spearheads Tim Southee and Trent Boult, who were rested, as Matt Henry captured four of the first give wickets. He had 4-17 after his first five overs and finished with 4-49.
"To be able to take 5-27 up front was a truly outstanding bowling performance," McCullum said. "Both Adam Milne and Matt Henry, back into the side with the new ball in their hand, were outstanding for us."
Milinda Siriwardana made 66 and Nuwan Kulasekara 58 in a 98-run seventh wicket partnership which saved the Sri Lanka innings from total collapse but they were out within five balls of each other and Doug Bracewell (3-37) and Mitchell McClenaghan (2-40) quickly wrapped up the tail.
Siriwardana showed great fighting qualities in reaching his second half century and highest score in one-day internationals, remaining composed when he came to the crease with the Sri Lanka innings in disarray. He also had the consolation of picking up both McCullum and Guptill's wickets, the latter in combination with Kulasekara.
Kulasekara played an equal role in a spirited fightback, producing a 73-ball innings which included two fours and five sixes.
"We just had to bat well and put pressure on the Kiwis but unfortunately we just kept losing wickets in the first 10 overs when six of our batters were back in the pavilion," Mathews said. "It was tough going but it was a great effort by Milinda Siriwardana and Nuwan Kulasekara."
BRIEF SCORES (FULL SCORECARD)
Sri Lanka: 188 in 47 overs (Milinda Siriwardana 66, Nuwan Kulasekara 58; Matt Henry 4/49, Doug Bracewell 3/37, Mitchell McClenaghan 2/40) lost to New Zealand: 191 for 3 in 21 overs (Martin Guptill 79, Brendon McCullum 55; Milinda Siriwardana 2/45) by 7 wickets.