Chasing just 126 for victory, his team slumped to 59-6 before edging home and captain Chalmers said: "We don't make life easy for ourselves.
"I was more than happy at half time because I thought we bowled really well.
"But our batsmen didn't play the conditions at all well. We went for shots that were completely unnecessary.
"We knew Dumfries would make it hard because that's the sort of team they are and they never gave up.
"At 59-6 I have to admit I was feeling the nerves but Ewan played a fantastic innings and got us the win It's a great feeling."
It looked as if the contest might be a mis-match when Dumfries, were sent in to bat and saw wickets fall at an alarming rate to reach 26-5.
Nerves may have played a part in the collapse and that was certainly how it appeared when a needless run-out accounted for Joe Clarke and sparked the crisis.
However, there was also some accurate bowling from the Watsonians bowlers who exercised control from the start. Michael Legget, in particular, used the conditions to his advantage, while the Scotland Under 19 skipper Paddy Sadler also put the ball on a good line and length.
However, the Western Premier champions showed their renowned fighting spirit to make a recovery and end up posting a creditable 125.
They then threatened to pull off the most remarkable victory as nerves jangled in the 'Sonians ranks.
However, Ewan Chalmers was a picture of calm as he waited for the bad deliveries and dispatched them unerringly for priceless runs before his side sealed a three-wicket success with 11 overs to spare.
The Scotland batsman had walked to the crease with the score on 11-2 after the evergreen Pat Druce, one of several seasoned campaigners in the Dumfries ranks, and Chris McCutcheon had accounted for James Easton and Gus Fraser-Harris.
The international watched further mayhem unfold as Andy Hislop became Druce's second victim before his skipper fell to a fine catch by Druce off Scott Beveridge.
Left-armer Beveridge looked capable of turning the match the underdogs' way when he then accounted for Andrew Chalmers and Craig Wright in the same over on his way to figures of 3-17.
But Ewan Chalmers found a stoic partner in Pete Legget whose 22 helped add 62 decisive runs for the seventh wicket. By the time Pete Legget was trapped in front Watsonians were three short of their target and Ewan Chalmers was unbeaten on 70.
The talented Scotland man, stroked eight fours on a day when boundaries were a rarity.
Despite the disappointment of defeat, Dumfries captain Stuart Corbett-Byers insisted: "We've had a great season and the guys can be proud of their efforts.
"We never give up and we fought back well to put them in a bit of trouble."