Born in Madrid, Spain, Herrero bought his first motorcycle at the age of 12. In 1962, he obtained his racing license, competing on a Derbi and doing his own maintenance. He soon moved up to a Bultaco Tralla 125 and caught the eye of Luis Bejarano, the owner of Lube (a Spanish motorcycle marque) who recognized Herrero's talent. Bejarano offered him a job in the company's competition department. In 1964, Herrero finished in third place in the 125cc Spanish National Championship and in 1965, he finished in second. Unfortunately for Lube, the marque ran into financial difficulties and went out of business.

Herrero decided to go into business for himself, running a motorcycle repair shop in Bilbao. He purchased a Bultaco and competed as a privateer. Around this period, Eduardo Giró, lead designer of the Ossa motorcycle company developed a revolutionary bike with a monocoque chassis. Recognizing Herrero's riding talent as well as his mechanical skills, Giró offered him a job to develop the Ossa 250cc race bike. Together they won the 250cc Spanish National Championship in 1967.

In 1968, he would move up to compete in the 250cc Grand Prix world championship. Although the single cylinder Ossa had 20 horsepower (15 kW) less than the powerful V4 Yamahas of Phil Read and Bill Ivy, the Ossa was 45 pounds (20 kg) lighter and its monocoque frame was much stiffer, giving it superior agility. The Yamahas swept the championship, but Herrero left no doubt that the little Ossa was quick and dependable. He finished seventh in the championship and claimed a third place in the final race of the season at Monza. He would once again take the 250cc Spanish National Championship.

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