Craig Noone's headed effort just shy of the hour was enough to earn City the three points and the trophy with three games to spare, but only after enduring a tense finale.
City thought the points were wrapped up when Jamie Maclaren netted with three minutes to play, only for the hosts' striker to be denied when a VAR (video assistant referee) check found he had handballed in the build-up when tussling with Kye Rowles.
The Mariners then thought it had a chance to snatch a point in stoppage-time when goalkeeper Mark Birighitti – up at a late corner – won the visitors a penalty.
But once again, VAR intervened with referee Shaun Evans reversing his original call after checking the pitch-side monitor, to leave the home fans breathing a sigh of relief and celebrating the confirmation City will finish top at the end of the regular season.
Elsewhere, Wellington Phoenix dazzled on its New Zealand homecoming with a 3-0 win over Western United that keeps it on the coattails of the A-League's top six.
It had been 433 days since Phoenix had played a home fixture at Sky Stadium because of the coronavirus pandemic and a bumper crowd of 24,105 was not disappointed.
Clayton Lewis's deflected 38th-minute opener had Wellington in front at the break and two goals in the space of six second-half minutes from Reno Piscopo and Tomer Hemed had the patient crowd celebrating a comfortable win.
While having the fans back in attendance was a victory itself, the win means Phoenix are just three points shy of Brisbane Roar in sixth with three games to play before the A-League Finals.