In 1977, Leonard was at the peak of his powers. He became the second athlete to run the 100 metres in less than 10 seconds with electronic timing, running in 9.98 s on August 11 in Guadalajara (the first was 1968 Olympic champion Jim Hines), nowadays Leonard stands as the only Spanish-speaking 10-second barrier athlete. Leonard also had good speed endurance which he proved when he was the fastest in the world that year in the 200 metres with a time of 20.08 s. At the inaugural Athletics World Cup that year, Leonard won bronze in both the 100 and 200 metres whilst representing the Americas. On 13 September he set a low-altitude world best time for the 100 m at 10.03 s. He also won the 1977 100-metre Gold at the World Student Games.