The home team took control from beginning of their innings, with Bradley Scriven (32) and Michael Burgess (59) looking particularly assured in their 81-run partnership for the second wicket.
Although Ashtead hoped that the tables had turned when Tom Homes trapped Scriven leg-before, they knew that they would need to secure the wicket of the dangerous Danny Miller at a cheap price to push for the victory.
That was not to be. Miller instead headlined the innings with an all-out assault on the Stags' bowlers, finishing with 102 off just 70 balls. He swiftly put the result beyond doubt, pumping the ball to all corners of the park.
With able support by Burgess and Aaron Phillips, Miller led his team to victory with nearly 12 overs to spare.
Earlier, Ashtead's batsmen had also enjoyed the tremendous batting conditions. Michael Sanderson (70) and Stewart Cameron (87) compiled a solid 104-run partnership in the middle of the innings, and John Vaughan-Davies whacked a rapid 50 runs off 36 balls to take Ashtead to 250.
But when Cheam batted, it became evident that Ashtead had failed to take equal advantage of what was a certifiable road with minimal assistance for the bowlers.
Ultimately, that proved the difference between the teams in this encounter.
Next week Ashtead face Old Whitgiftians at Woodfield Lane.