Juantorena was born in Santiago de Cuba. As a 188 cm (6 ft 2 in) 14-year-old, he was first considered a potential star at basketball and was sent to a state basketball school. A talented athlete at many sports, he had been a regional high-school champion at 800 and 1500 meters, but his true talent at athletics was discovered by a Polish track coach, Zygmunt Zabierzowski, who convinced him to start running seriously. Juantorena was ready for the change because as he states himself he was a 'bad' basketball player and his idol when young was an athlete, the Cuban sprinter Enrique Figuerola. Only a year later, Juantorena was eliminated in the semi-finals of the 400 m event at the Munich Olympics (1972).
In the next years Juantorena won a gold medal at the World University Games (1973) and a silver at the 1975 Pan American Games, both in the 400 meters. He was unbeaten in 1973 and 1974, but underwent two operations on his foot in 1975. He only seriously took up running the 800 meters in 1976, so few thought he was a candidate for the Olympic gold that year. His coach, Zabierzowski, had initially tricked him in to trying an 800 race by convincing him the other runners need a pacemaker. However, Juantorena made it to the Olympic final, and led the field for most of the race, eventually winning in a world record time of 1:43.50. He was the first non-English speaking athlete to win Olympic gold in this event. Three days later, he also won the 400 meter final, setting a low-altitude world record of 44.26. By winning the 400 meters, he became the first athlete since Paul Pilgrim at the 1906 Intercalated Games to do such a double at an Olympic sports event, and was the first to do so at an officially recognized Olympics.
In 1977, he set another world record in the 800, running 1:43.44 in Sofia at the World University Games. He also won both the 400 m and 800 m at the 1977 IAAF Athletics World Cup in Düsseldorf, Germany. The 400 m race was mired in controversy when the race was re-run a day after the initial race, in which Juantorena finished third, because Juantorena lodged a successful protest that his slow start had been due to not being able to hear the starter's gun. The latter race featured an epic duel with his great rival Kenya's Mike Boit, a duel that did not happen at the previous year's Olympics because of the African countries boycott.
Juantorena, now known at home as El Caballo (the horse), continued his career, although injuries meant he would never reach the same level as in Montreal. Juantorena had been born with flat feet that caused feet and back problems, and he had to have corrective surgery in 1977. In 1978 he was unbeaten at the 400 m, but suffered his first ever defeat at 800 meters. Injuries, particularly hamstring injuries, hampered his training and racing leading up to the 1980 Moscow Olympics, where he just missed out on a medal in the 400 meters, placing fourth. At the 1983 World Championships, his last international appearance in a major event, he broke his foot and tore ligaments when he stepped on the inside of the track after qualifying in the first round of the 800 m. As Juantorena was being carried off the track on a stretcher most viewers must have thought that was the end, but Juantorena returned to training with a view to competing in the 1984 Los Angeles Olympics. However, when Cuba joined the boycott of those Olympics his last chance of international glory was ended.
After retirement from athletics in 1984, Juantorena has served in many official capacities, including as the Vice President of the National Institute for Sports, Physical Education and Recreation for Cuba, Vice Minister for Sport of Cuba, and Vice-President, later Senior Vice-President of the Cuban Olympic Committee.
He also married Yria, a former gymnast, and is still a keen fun runner, running c. 10 kilometers a day and participating in marathons.
Juantorena has also been appointed as a member of the IAAF Council, and has also served as an Athletes' Commission Chairman and Grand Prix Commission Member.
Juantorena was ranked among the best in the world in both the 400 and 800 m sprint events over the incredible spread of 10 seasons from 1973 to 1982, according to the votes of the experts of Track and Field News.