As it had been in Saturday's Qualification Race, it was Marciello against a pair of Porsches, Laurens Vanthoor leading the charge for the Rowe Racing squad after getting past teammate Earl Bamber on the opening lap.
Vanthoor and Bamber were initially unable to do much about the leading Mercedes, Marciello holding an advantage to the tune of eight-tenths over the first four laps.
At that point the Porsches were handed a second chance at attack Marciello, the race neutralised by the Safety Car after sixth-placed Kevin Estre hit the wall at Mandarin.
The stoppage set up a thrilling Lap 8 restart, Vanthoor brushing the back of Marciello at Lisboa Bend as he made a bold, but ultimately unsuccessful, play for the lead.
At the same time Bamber came under fire from Augusto Farfus Jr. who, having been more than four seconds behind the leader before the Safety Car, used his M6's superior straight-line speed to briefly pull clear of the third-placed Porsche on the run to Lisboa Bend.
It was only a brave move from Bamber into the iconic right-hander that restored the top three order, Farfus then dropping more than three seconds on the restart lap alone.
From there it was a similar story as the first stint, Vanthoor stalking the Mercedes but seemingly unable to mount a genuine challenge for the lead.
On Lap 13, with the gap now over a second, the Rowe Porsches swapped positions to give Bamber a shot at Marciello. The Kiwi quickly closed to within a second, only for the gap to blow back out to 1.5 seconds after a mirror-smashing glance of the wall at Police with a few laps to go. He then made one more charge on the final lap, getting close enough to nudge the back of Marciello's AMG at the Melco Hairpin.
But there was no way through, Bamber conceding defeat at 'R' Bend and slowing to let Bamber move back into second place.
"I'm really happy that I've finally won here at Macau," said Marciello. "I've always been close. I was on pole in Formula 3 and last year in the GT, but Macao is Macao. It's really special, everything needs to be perfect. And finally, I did it. It's an amazing feeling to race against the best GT drivers in the world, and to win this trophy means that I can be one of the best."
As for the last lap battle with Bamber, Marciello added: "At Macao it's difficult to overtake, so he had to try something a bit crazy to pass me. Luckily my car was good in the last sector all weekend. We lacked a bit of braking performance, but the last part of the track was mega for us. So, it was hard, but I knew everything was in my control and I was able to bring it home."
Farfus ended up best of the rest in fourth, the margin a crushing 23 seconds, followed by Audi driver Chris Haase and Edoardo Mortara, who put in an impressive charge from the back end of the grid after his Saturday crash. Joel Eriksson was seventh ahead of Dries Vanthoor, Maro Engel and Kelvin van der Linde.