Sir Dragonet defeated former stablemate Armory to land the 100th running the of the Cox Plate at Moonee Valley on Saturday.
Trained jointly by Ciaron Maher and David Eustace, Sir Dragonet left the care of Aidan O'Brien after owner Ozzie Kheir acquired the son of Camelot in August.
The mount of Glen Boss, Sir Dragonet was sent off 13-2 for the prestigious weight-for-age contest over 10 furlongs, despite a losing run that stretched back to the 2019 Chester Vase.
That last win came on rain-softened ground on a track with a tight configuration, and the four-year-old experienced a similar set of prevailing conditions as he broke slowly from stall six in the 14-runner Group One contest.
Boss was content to bide his time towards the rear of midfield for much of the trip, as outsider Grandslam led until just before the home bend.
Russian Camelot moved smoothly into contention and briefly took the lead with two furlongs to run, with Armory and Mugatoo also looming large.
Sir Dragonet was angled off the rail by Boss as they approached the straight and his rivals had no answer as he powered away in the final furlong to score by a length-and-a-half, with Armory (13-2) outstaying Russian Camelot (5-2F) for second place.
"This is unbelievable," Boss said of his fourth Cox Plate success.
"I couldn't have scripted the run any better. He was up for the task."
Joseph O'Brien's Buckhurst finished 10th, one place ahead of Roger Charlton's Aspetar, while Aidan O'Brien's Magic Wand was a non-runner due to lameness.