In 1934 Somes received the first scholarship given to a male by the Sadler's Wells School, and in 1935 he joined the ballet company. By 1937 he was appearing in solo parts; his first major creation was in Frederick Ashton's Horoscope (1938). After serving in World War II, he returned to create additional important roles in ballets choreographed by Ashton, including Symphonic Variations (1946), Cinderella (1948), Daphnis and Chloë (1951), Tiresias (1951), and Ondine (1958). He also performed in such classical ballets as Swan Lake, Giselle, and The Sleeping Beauty.

In 1950 Somes succeeded Robert Helpmann as Margot Fonteyn's official partner, and in 1959 he was made a Commander of the Order of the British Empire. After retiring as premier danseur (1961), he became assistant director of the Royal Ballet in 1963 and appeared in pantomime roles such as the Father in Marguerite and Armand (1963). Leaving his assistant directorship in 1970, he remained with the Royal Ballet as its principal teacher until 1984.