Castellotti was born in Lodi, Italy. He acquired a Ferrari at the age of twenty, from a local benefactor, and began racing sports cars in 1952. That year he won the Portuguese Grand Prix, was third at Bari and second at Monaco which was run that year for sports cars. In 1953 he won the 10 Hours of Messina and finished third in the Carrera Panamericana in Mexico. In 1954, he signed for Lancia and again drove sports cars whilst awaiting the team's Grand Prix car. He eventually made his Grand Prix debut at Buenos Aires on 16 January 1955, for Lancia, but struggled in the warmer temperatures and crashed. However, he finished second at Monaco, but in mid-season the team amalgamated with Scuderia Ferrari, for whom Castellotti drove for the remainder of his career. He participated in 14 World Championship Grands Prix, achieving 3 podiums and scored a total of 19.5 championship points. He secured pole position, with Lancia, at the 1955 Belgian Grand Prix, becoming the youngest driver to do so (at age 24 years, 7 months and 26 days), a record that stood for 13 years until Jacky Ickx's pole position at the 1968 German Grand Prix.