McDonald went into the meeting with rides on a handful of the leading fancies, including odds-on Golden Rose favourite Anamoe.
The top jockey didn't let it get to him, turning around his fortunes when he produced Head Of State ($5.60) with a late surge down the outside to round up Profondo ($4.60 fav) and claim the Group 3 Gloaming Stakes (1800m) by a neck.
McDonald rated it a terrific performance, especially given it had been hard to make ground on the firm track.
"There's still more to come. He loves that ground," McDonald said.
"A huge effort against the bias."
Profondo ducked back to the fence and may have cost himself victory, his effort at just his second start auguring well for his future.
The pair cleared out from the rest with Benaud ($21) third, another 3-3/4 lengths back.
Head Of State is by champion American Pharaoh and trainer Chris Waller is convinced he can make his mark as a stallion if his progeny is given time to mature.
The three-year-old will have his next start in the Group 1 Spring Champion Stakes (2000m) at Randwick in two weeks.
"That's the plan. We thought he would take good progress from today's run in two weeks' time," Waller said.
"Obviously, the second horse was very good and he'll have a bit of improvement to come. I love the way they gapped the rest."