UK-born Mertens started his engineering career at British Aerospace, while racing himself in Formula Ford. In 1976, he joined March Engineering, at the time one of the world's largest manufacturers of race cars, where he rose to the position of Chief Designer working on F1, F2, F3 and IndyCar designs. In 1988 he started his own company, Galmer Engineering. The company was formed to provide a fully operational research and development facility for the Galles Racing IndyCar team.