The 22-year-old Arsenal fan birdied the last two holes to claim a breathless one-stroke win in front of anther packed house at Hillside after Wallace’s putter went cold on him at just the wrong moment.
The Englishman has carved out a reputation as a clinical closer but he missed short-range birdie putts at the 17th and another decent chance at the 18th where Kinhult punished him from 12 feet.
The calmer outlook Wallace had adopted ahead of the tournament duly went out of the window as he stormed off livid after his 71.
“It felt like Matt could have shot five under today because he played that well but he couldn’t really hole out,” said Kinhult, a Lytham Trophy winner in his amateur career.
“It was so close all day between us. It was a bit like a matchplay. Over that last putt my knees were shaking, my hands were shaking, everything was shaking. I don’t know how I managed it but somehow I did. It was a great birdie.
“I’ve been dreaming about this for such a long time and to finally do it was amazing. I hope it is the first of many.”
A charging Eddie Pepperell was also joint runner-up after a 66 which equalled the lowest of the day. Pepperell, bidding to become the first man since Greg Norman to win back-to-back British Masters, had come from five shots back overnight.
“It was a good round,” said Pepperell. “I expected those guys to get to 17 under and they got to 16 in the end and I wasn’t far off that but fair play to Marcus. He’s been a good player for a while and I’m a fan of his swing, so it was good to see.”
Scottish left hander Rob MacIntyre muscled his way to share of second too to record the best finish of his European Tour career after a spectacular eagle/birdie finish.
Tournament host Tommy Fleetwood ran out of steam with a 73 but drew satisfaction from the success of an event which drew more than 60,000 fans over the four days.
Next year the British Masters will move back to Close House, near Newcastle, where it was last staged in 2017.
It will have a new slot in the schedule – July 30 – and a different host with Lee Westwood resuming the duties at the venue where he is touring professional.
“Hopefully we will get another great field and I am sure it will be a great spectacle like it was a couple of years ago. The crowds in the north-east love their sport and their big events, and Close House will be at its best again,” said Westwood, who finished 35th at Hillside after tailing off with a 73 yesterday.