After the war, he studied singing, first with William Steven at the University of Alabama in Tuscaloosa, where he received the Bachelor of Music. More constructive studies then followed with Marie Sundelius at the New England Conservatory in Boston. After two years of study, Cox graduated with a Master of Arts with honors.
In 1951, his stage debut was at the New England Opera Boston as Lenski. He was awarded a Fulbright scholarship to study in Rome with Maestro Luigi Ricci. In 1954, he sang at the Spoleto Festival Rodolfo. Cox remained in Europe, where he began his career at various opera houses.
Privately, he studied in Munich with Max Lorenz.
During the season 1954/1955, he was hired as a lyric tenor by the Kiel Opera. In the province, he first sang many lyric-dramatic roles, including the title role of Serse, Cavaradossi, Stewa, Matteo and Lyonel. It was followed by another engagement at the Staatstheater Braunschweig (1955–1959), where he first sang the title roles in Lohengrin and Otello.
In 1959, he was hired by the National Theatre Mannheim. He was there without interruption for nearly 40 years as a permanent member. His debut at the National Theatre Mannheim was in 1959 as Alfredo. During his career, Cox sang in Mannheim almost all the tenor repertory both for lyric tenor and heldentenor.
In Mannheim, Cox developed continuously, starting with the Italian roles to the heroic tenor roles. His repertory included: Rodolfo, Duca, Radames, Calaf, Max, Otello, Florestan, Wagner roles, ...
In 1977, the city of Mannheim appointed Cox as Kammersäger. Later in his career, Cox changed to character roles. In 1989, he sang Captain Vere at the National Theatre. In 1996 he took, at the age of 74, his last performance as Aegisthus.
Already in 1956, he first appeared at the Bayreuth festival as Steuermann in Der fliegende Holländer. From 1967 to 1975, he sang every year. He sang Lohengrin (1967, 1968), Walther (1968–1970, 1974, 1975), Parsifal (1968, 1973), Erik (1969), Siegfried (1970–1975). In later years, Cox sang in 1978 as Siegfried (Siegfried), in 1983 Siegfried (Götterdämmerung) and again in 1984 as Walther.
Often, Cox saved a performance at the last minute avoiding a cancellation.
In 1961, he sang at the Bregenz festival (in Fra Diavolo and in the operetta Die Trauminsel by Robert Stolz) and at the Teatro Nacional de São Carlos, where he sang Pylade in Iphigénie en Tauride.
From 1963 to 1977, Cox sang regularly at the Vienna State Opera. He made his debut there on October 23rd, 1963 as Bacchus. He performed in 75 performances, including Bacchus, Apollo, Turiddu, Stewa, Sergej (Lady Macbeth of Mtsensk), Hoffmann, Erik, Lohengrin, Stolzing, Parsifal, Siegmund, Siegfried (Siegfried), Sänger and Tristan. His last appearance was as Bacchus on January 18, 1977.
He sang at the Vienna Volksoper (December 1963 as Carlo Moor in Masnadieri, and Sándor Barinkay), at the Hamburg State Opera (regularly 1958–1973), at the Lyric Opera in Chicago (1964, 1970 and 1973 as Bacchus, Siegfried and Erik), the festival of Aix-en-Provence (1966 Bacchus), the Bavarian State Opera (1967, and in 1969 as Siegfried), at the Opéra in Paris (1971 and 1972 as Siegmund), at Covent Garden (1975, debut as Siegfried), at La Scala (1975 as Siegfried) and at the Met (April 1976, Walther).