Dennis was born in Sidcup, South London, and was an enthusiastic sports car and Formula Junior racer. He started racing in 1951 with an Iota 500cc F3 car and had long and successful career in F3. He drove a variety of 500cc machines including JBS, Arnott, Martin, Staride and Cooper.
In 1958 to moved to Formula 2 driving a Lotus 12 in British F2 races. He entered the British Grand Prix in 1959 but failed to qualify.
When F. Junior began in Europe he acquired a front-engined Lola for 1960 season. The following year he bought a new rear-engined Lola.
In 1962 during the F. Junior race at Monaco on Saturday of F1 Grand Prix he crashed the Lola with fatal consequences.