The black dog was one of two finalists for Victorian trainer Jason Thompson’s whose other runner, Invictus Rapid (box four), shot through to take the early lead when the lids flew open.
Buck Forty was able to settle in second position from box two after gaining a beautiful run through along the fence, giving the leader four lengths in the run through the catching pen.
Down the back and the son of Buck Fever and Fancy Jaffa began grinding away, slowly taking ground off Invictus Rapid, before railing underneath his kennel mate around the home turn.
Upon straightening and the two Thompson-trained chasers were going stride for stride, with Buck Forty going home the better to score by half a length in 29.83.
Race favourite Ritza Lenny finished third for Mark Gatt after stumbling at the start, with Shima Song back in fourth spot for Andrea Dailly.
It was the biggest career win for Buck Forty to date, with the Peter Mosman Classic adding to his previous victory in the Group 2 Launching Pad at Sandown Park.
The 37 kilogram heavyweight has now won eight of his 21 career starts, with $263,388 in prize money earnings.
Watch Buck Forty (2) win the Group 1 Peter Mosman Classic.