Just five of the 34 starters made it into the jump-off, with Wachman posting a winning time of 32.59 seconds to take the winner's prize of €27,500.
That left the young Tipperary man just over a second ahead of Canadian runner-up Kara Chad with Igor GPH, while Austria's Max Kuhner finished third with Greece 5.
Kuhner had taken victory in London's Longines FEI World Cup Grand Prix 24 hours earlier with the Irish-bred EIC Cooley Jump the Q.
Wachman explained how he aimed his horse specifically at the Grand Prix, which closed out five days of show jumping action at London International.
"He [Do It Easy] is a brilliant horse and we just thought this class would suit him. He has been in great form the last few weeks, so I'm just delighted with the way he jumped," he said.
"Obviously, there is a big team that goes into it, so it's great for all of them. It's [London International] such a traditional show, a historic show, I am just delighted to come here and delighted to win."
Just last month, Tom Wachman was named the winner of the 2025 Longines Rising Star Award by the sports international governing body, the FEI.
A three-time European champion at Youth level, he has won team gold in the Pony (2019), Junior (2023), and Young Rider (2024) categories, and helped Ireland secure team silver at the FEI Jumping European Championship for Young Riders, Juniors & Children 2025 in Riesenbeck, Germany.