Bruce was the one of the best drivers in South Africa at the end of the Fifties and the beginning of the Sixties.
Bruce raced a Volvo before entering single-seater competition. Bruce was first noticed by finishing in 6th place at the Formula Libre South African Grand Prix in 1960 won by Paul Frère (Who shares the same birthday though 10 years his senior).
The following year, at the wheel of Cooper with an Alfa Roméo engine, he took second place in the South Africa Formula 1 Championship. It all came down to the last race, the Rand Spring Trophy at the then brand new Kyalami circuit. The situation was pretty straight forward, to win Bruce Johnstone needed to win the race, otherwise it was match point to Syd van der Vyver. Ernie Pieterse, a teammate of Johnstone on Scuderia Alfa, took the pole with van der Vyver and Johnstone filling out the front row. On the sixth lap the Championship was handed to van der Vyver as Johnstone had a tire blow and had to retire. Pieterse won the race with John Love second and van der Vyver third. Although both scored maximum points 24 points - scores from only three of 10 events counted, van der Vyver won the tie-breaker with both more wins and more gross points.
In finishing second however, he earned an invitation to drive for the Yeoman Credit team, under the stewardship of Reg Parnell. It was not a happy partnership, however, as Johnstone crashed the car both in the Natal GP and then in practice for the South African GP, rendering him a non-starter.
Bruce came to Europe briefly in 1962 and drove for Ian Walker, winning the up to 1.5 litre class in the Nürburgring 1000 Km with Peter Ashdown in a Lotus 23, and taking fifth in the Vanwall Trophy at Snetterton with Walker's Formula Junior Lotus. He also drove for the Owen Organisation with team mates Graham Hill and Richie Ginther, in a works BRM in the Gold Cup race at Oulton Park, finishing fourth, and took the same car to ninth place in his home Grand Prix at the East London track later that year.
Nevertheless, Bruce then gave up single-seaters and turned to sports cars winning the 1962 Rand 9 Hours at Kyalami sharing David Piper's Ferrari 250 GTO that year.