Prieto was born in Segovia, and made his international debut at the 1983 European Athletics Junior Championships, placing fifth in the 10,000 m walk. He came seventh in the 20 km race walk at the 1985 IAAF World Race Walking Cup then went on to win a bronze medal in the event at the 1986 European Athletics Championships. He established himself as one of Spain's leading walkers, competing at the World Championships in Athletics in 1987 and 1991. Prieto won his second international medal at the 1989 Summer Universiade, where he was the silver medallist in the 20 km walk behind Italy's Walter Arena. He came 13th at the 1990 European Athletics Championships and finished one place lower than that at the 1991 IAAF World Race Walking Cup.