He was coach of the Spain national basketball team for as long as 27 years (1965–1992). In more than 300 games, Díaz-Miguel guided the Spanish National Team to six Olympic games, including a silver medal in the 1984 Olympic Games in Los Angeles, the top classification for the Spanish National Team until 2008, when they finished second again. Díaz-Miguel's Spanish National teams participated in 14 European Championships and four World Championships, and his team's best finishes were two silver medals (1973, 1983) and one bronze medal (1991) in the European Championships.
He was named "Best Spanish Coach of the year" in 1981 and 1982. Díaz-Miguel coached six times European All-Star teams, created only for very occasional events in Europe. A pioneer in the worldwide promotion of basketball, he won numerous coaching awards and was a frequent basketball lecturer around the world. He was enshrined in the Basketball Hall of Fame as a coach on September 29, 1997. He is the only Spaniard in the Hall. In 2007, he was enshrined in the FIBA Hall of Fame.
Although initially a soccer player in his youth, his physical characteristics, including a 6 ft 1 in (1.85 m) height, and also determination contributed to his start in basketball at the Instituto Ramiro de Maeztu Secondary School of Madrid, where the Estudiantes ("students") Club was founded in 1950 with pupils and teachers of the School. He was a player of Estudiantes Club of Madrid in 1950-52 and 1953–1958, and played later also for Real Madrid (1958–1961) and Águilas of Bilbao (1961–1963). He was 27 times player of the Spanish National Team. He won the Spanish Clubs League Championship twice, in 1959 and 1960 with Real Madrid. Immediately after retirement, he started coaching Águilas, becoming afterwards provisionally and nearly by chance coach of the Spanish National Team, where he managed to develop a longstanding sensation for the team and basketball in Spain.