Mantripp began racing in 2023 at age nine in the GT15 class, a monohull powerboat class open to competitors aged 9 to 16.[5] He achieved a second-place finish in only his third competitive race.[5]
In November 2023, Mantripp competed at Coniston Records Week on Coniston Water in the Lake District.[6] The event is held annually to honour the memory of Donald Campbell, who broke eight world speed records on water and land during the 1950s and 1960s.[1] Mantripp set a GT15 class speed record of 39.16 mph (63.00 km/h), breaking his own earlier record of 38.13 mph (61.37 km/h) from the same week.[1][7] He became the youngest competitor in the 51-year history of Records Week to set and break a speed record.[1] For this achievement, he received the K7 Silver Star, named after Campbell's iconic Bluebird K7 boat, and was awarded The English Lakes Hotel Trophy as the week's youngest record breaker.[1]
On 7–8 June 2025, Mantripp won the GT15 UK Masters Championship at Stewartby Lake, Bedfordshire.[4] He won four consecutive heats in windy and challenging conditions to claim his first major title at age 11.[4] The victory was particularly notable as Mantripp had overcome a difficult start to the season, including a fourth-place finish at Oulton Broad in May after a collision damaged his Povvat hull.[4]
Mantripp continued to compete in the British Powerboat Championships throughout 2025.[8]