The 25-year-old, from Heriot, was part of the men’s quadruple sculls line-up that won the silver medal in the final, narrowly beaten by the Netherlands.
The British squad had passed a stern test to qualify via repechage and went on to secure the nation’s first medal in the men’s quad sculls since it was introduced to the Olympics at Montreal in 1976.
Leask and team-mates Angus Groom, Tom Barras and Jack Beaumont held their nerve to make history on the Sea Forest Waterway, stopping the clock in 5:33.75.
Australia won bronze, finishing along with fourth-place Poland within a second of the British boat.
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Leask, now based in Henley-on-Thames in Oxfordshire, said: “I knew basically the whole way where we were. I had a pretty good view from where I was sitting of the whole race.
“We just did what we wanted to in that first half. We got out well and I was looking across and seeing us in a really good position. It was really encouraging and it just feels amazing.”
The Team GB quartet produced a flying start from the unfavoured lane one and were rewarded for their bravery, going out hard and leading the race in the opening stages to earn their place on the podium.
Before the tournament, Leask had told the Southern Reporter he was “delighted, amazed, nervous and proud” to be selected for Tokyo. “Think of every emotion and it’s there,” he added. “I can’t wait to get out there and hopefully bring a medal home with the other three in the boat. Olympic selection and, hopefully, a medal, is what I have driven towards and dreamed of for the last 11 years.”
His aunt, Janet Kennedy, added on Wednesday: “It was a fantastic race. I’m over the moon.”
In addition, Maddie Arlett, of Selkirk, was reserve rower for Team GB’s pairing of Imogen Grant and Emily Craig, who reached the final of the lightweight women’s double sculls.
Earlier, Jedburgh swimmer Lucy Hope, after having her preparations hindered by Covid-19, was part of the women’s 4x100m freestyle relay crew that set a new UK record while finishing fifth in their final.