James Clark Jr was born into a farming family at Kilmany House Farm, Fife, the youngest child of five, and the only boy. In 1942 the family moved to Edington Mains Farm, near Duns, Berwickshire, in the Borders. He was educated at primary schools, first in Kilmany and then in Chirnside, and then following three years of preparatory schooling at Clifton Hall School near Edinburgh he was sent to Loretto School in Musselburgh, near Edinburgh.
Although his parents were opposed to the idea, Clark started his racing in local road rally and hill climb events driving his own Sunbeam-Talbot, and proved a fearsome competitor right from the start. On 16 June 1956, in his very first event, he was behind the wheel of a DKW sonderklasse at Crimond, Scotland. By 1958, Clark was driving for the local Border Reivers team, racing Jaguar D-types and Porsches in national events, and winning 18 races.
Then on Boxing Day 1958, Clark raced against the man who would launch him to superstardom. Driving a Lotus Elite, he finished second to Colin Chapman in a ten-lap GT race at Brands Hatch. In 1959 he drove a Lotus Elite, finishing tenth at Le Mans partnered with John Whitmore, and the ex-Bruce Halford Lister Jaguar, winning the Bo'ness Hill Climb. Chapman was sufficiently impressed to give Clark a ride in one of his Formula Junior cars.
"In March 1960, the first race for the newly introduced Formula Junior took place at Goodwood. The winner was J. Clark (Lotus Ford), from J. Surtees (Cooper B.M.C) with T. Taylor (Lotus Ford) third."
Clark had made an earlier FJ appearance in a one-off race at Brands Hatch on Boxing Day, 1959, driving a Gemini-B.M.C. for Graham Warner of the Chequered Flag garage, Chiswick.