Reynolds went on to collect a third win of the campaign by 4.6807s, but McLaughlin was the big story.
The 25-year-old started the race with a 53-point lead over fellow Kiwi van Gisbergen after the Red Bull Holden Racing Team driver was penalised for a Saturday pitstop infringement.
While he only had to finish inside the top six, McLaughlin looked after his own fortunes to ensure the Ford Falcon signed off with a title.
It is an eighth drivers' championship for Dick Johnson's operation, and the first since Team Penske bought into the squad four years ago.
"Everyone here work so hard," McLaughlin said.
"We have to pay respect to Shane, we raced so hard all year, it’s been a hell of a battle.
"With 30 to go I thought ‘this could be on’ and it was the longest 30 laps of my life.
"Holy smokes, I just can’t believe we’ve done it. Holy guacamole."
Van Gisbergen could not fire a shot in the race and finished a distant fourth, with team-mate Jamie Whincup claiming the final podium position.
The final margin at the top of the championship was 71 points in McLaughlin's favour.
Craig Lowndes, meanwhile, bowed out of full-time Supercars competition with an 11th-placed finish.
Polesitter Reynolds kept the lead off the line and headed McLaughlin by just over a second after 10 laps, with van Gisbergen another second back in third.
Van Gisbergen lost a place to Garth Tander at the start but got back past the Garry Rogers Motorsport Commodore at Turn 8 on the opening lap.
The protagonists were still split by a second, but had fallen slightly further behind Reynolds' Holden when the Safety Car was called on lap 20.
That was for the second Turn 12 clash in as many days between Jamie Whincup and Fabian Coulthard, which left the latter's Falcon without a rear wing.
It prompted a flurry of pitstops in which McLaughlin jumped Reynolds, while a slightly-earlier stop helped Whincup vault up to third.
Van Gisbergen, meanwhile, lost two places to fifth, to Whincup and James Courtney, who had run seventh and fifth but pitted before the Safety Car period.
The next phase of the race was largely status quo, McLaughlin edging away to be 3.1s clear of Reynolds on lap 40, while van Gisbergen remained fifth, 8.2s back.
Bottled up behind Courtney, van Gisbergen's deficit to McLaughlin was out to 12.6s by the time he pitted for a second time on lap 47.
McLaughlin and Reynolds made their second stops five laps later, the gap between them reduced but still two seconds when the strategies played out.
Van Gisbergen did at least get past Courtney but was unable to pull away, and faded further to be 25s behind McLaughlin with 30 laps remaining.
At the front, McLaughlin kept a steady buffer over Reynolds for most of the final stint, but it was down to within a second into the final 10 laps.
Knowing second place would be more than enough to win the championship, McLaughlin and DJR Team Penske opted against getting into a fight with the Erebus man.
McLaughlin let Reynolds past on the run to Turn 2 with five laps remaining and ensured there was no repeat of his Newcastle 2017 heartbreak.
"I had a mega, mega car out there," Reynolds said.
"I got a really good start and first stint, my second stint wasn’t the best, Scott got away from us and we had a bit of fuel time on him to lose.
"The car got better and better, I could see him starting to slide and thought ‘if he’s smart, he’ll let me go’ and then he did.
"But I was lining up for a crack regardless, I’m here to win. He won the championship anyway, he’s done a great job all year and it couldn’t have gone to a better driver."
While the roles were effectively reversed in the Whincup/Coulthard clash to Saturday, the outcome was the same, stewards taking no further action.
Whincup continued and finished third, while Coulthard was ultimately classified 26th, 20 laps down, after initial repairs and another longer visit to pitlane late.
Van Gisbergen's afternoon was compounded by tyre pick-up damaging the guard above the left-front and then later right-front wheel.
He persisted and was able to keep Courtney at bay but crossed the line 42s behind Reynolds.
"We did everything we could. Congrats to them, they’ve been worthy competitors and worthy champions," van Gisbergen said.
"I’ve enjoyed the fight and am still proud, but it’s a tough one. That race, I just got stuck behind James and couldn’t do anything to get up there.
"I’m still happy, it’s still cool, I just feel a bit gutted that the series did what they did. It’s just inconsistent, it’s average.
"I would have loved to have fought on equal points and had a bit more motivation, but we were pretty dejected today with all that."
Andre Heimgartner claimed a career-best result of sixth, in the final race of Nissan's factory backing of Kelly Racing.
Chaz Mostert was the lead Tickford Racing driver home in seventh, while Tander, Lee Holdsworth in his Preston Hire Racing swansong and Scott Pye rounded out the top 10.
Lowndes led a handful of laps off-sequence before making his second pitstop, and emerged in 16th place.
He made progress back up the order on the run home on fresher rubber to finish 11th in his final Supercars race as a full-time driver.
Nick Percat was running sixth in the final stint but had to make an extra pitstop with 22 laps remaining, with his Brad Jones Racing Commodore having fallen just short of the minimum 140-litre fuel drop.
That extra visit to pitlane demoted Percat to 18th but he worked his way back up to 12th.
Mark Winterbottom was next 13th to cap his 13 seasons with Tickford Racing, while Anton De Pasquale was the lead rookie in 16th.