Richard Fahey’s four-year-old, with his former full-time jockey Paul Hanagan riding, returned to his early-season form in style as he extended his lead into a five-length victory.
The Cheka was the best of the rest, with 7-2 favourite Society Rock coming third.
Mayson was an outside tip for the race, having disappointed in his previous two outings at York and Newcastle.
"I was standing looking at the betting and he was the outsider of the lot and I was thinking 'this is the biggest joke in a long time',” Fahey, who now mostly rides for Hamdan Al Maktoum, said afterwards.
“He's back to his best which is good. At York he got very upset and got his back leg caught in the stalls and at Newcastle it was bottomless and he couldn't cope with it. He loves soft ground but not like at Newcastle.
"He's in the Nunthorpe but I'd have worries going back there after that experience but I'll speak to the owners and see."
The previous favourite, Bated Breath, was withdrawn due to the wet conditions.
It was Fahey and Hanagan’s first English Group One win together.
The heavy rain and withdrawal of Bated Breath boded well for an upset, and Mayson did not disappoint although he was helped by his rivals lack of fight in the conditions.
The Cheka did make a late attempt to haul him back, but Hanagan pulled further clear to romp home in style.