A Belgian born in Fulham, London, with a Scottish mother, Claes' first passion was jazz, but he became involved in motor racing after a chance visit to the 1947 French GP where his bilingualism allowed him to act as an interpreter to the English drivers.
His Belgian father was wealthy, and when Johnny tried his hand at racing in 1948, a Lago Talbot was duly ordered for him which saw much service in both 1949 and 1950. However, his first real success came at the wheel of an HWM in the 1950 GP des Frontières at Chimay, the first win in a Formula race for HMW. He won the race again the following season, this time in a Simca-Gordini.
He won the Liège-Rome-Liège Rally in 1951, driving a privately entered Jaguar XK120. It was the only time the rally was won without any incuring any penalties.
With his Talbot effectively redundant following the adoption of Formula 2 regulations for Grands Prix, Claes secured drives with the Gordini, HWM and Connaught teams during the 1952 and 1953 seasons.
Finding the competition tough at World Championship level, he tasted success again in the 1953 Liège-Rome-Liège Rally, which he won despite having to drive the car single-handed for 52 hours after his co-driver was taken ill. After the rally, Gianni Lancia presented him with a Grand Touring Lancia in recognition of the feat.
By 1954 Johnny was a sick man and he raced little, though a visit to Le Mans with a Porsche saw him finish 12th overall and take the 1500 cc class.
He was more active in 1955, the highlight of his season being fourth place at Le Mans in the Ecurie Belgique Jaguar with Jacques Swaters. His last competitive event was to be the Liège-Rome-Liège Rally of that year, in which he took third place partnered by Lucien Bianchi.
He was invited to drive for Standard in the Monte Carlo Rally but his health had deteriorated rapidly and, laid low by tuberculosis, he died in February 1956, aged just 39.