After qualifying as a mechanic Caldwell joined McLaren for the 1967 Italian Grand Prix. He then rose to be what would now be called the technical director of the team. Drivers he worked with included Emerson Fittipaldi, James Hunt and Jochen Mass. He was involved in the design, production and preparation of the cars for racing and was the inventor of an air-starter to make the cars lighter and also introduced a sixth gear, skirting to increase down force and team uniforms to replace the assortment of overalls and other clothes that were previously worn.