He was soon thrust into the international limelight when he finished 10th in his first appearance in the Le Mans 24 Hours in 1959, sharing an Elite with future World Champion and good friend Jim Clark.
He maintained a constant presence in endurance racing at home and abroad during the first half of the 1960s, demonstrating his versatility in AC, Aston Martin, Austin-Healey, Jaguar, Lotus, MG and Porsche machinery. He landed plum roles at Le Mans in a works Ford GT40 – in 1965 sharing with Innes Ireland and ’66 with Frank Gardner – but failed to finish on both occasions.
Whitmore also made a name for himself in touring car racing in Britain and on the Continent. He won the British Touring Car title in his first season aboard a BMC Mini Minor in 1961 and took the runner-up spot two years later, also in a Mini.
He also enjoyed success in the burgeoning European Touring Car Championship in 1964, winning at Zolder, Brands Hatch, Karlskoga, St Ursanne and Timmelsjoch in an Alan Mann Racing Ford Lotus Cortina.