Frantz began playing piano at the age of four and later studied with Eliza Hansen and Wilhelm Kempff under a scholarship from the Studienstiftung des Deutschen Volkes, or German National Scholarship Foundation. In 1967, he won an international musical competition hosted by a famous German television station. He first played with the Berlin Philharmonic Orchestra, conducted by Herbert von Karajan in 1970. In 1975, he played in his U.S debut concert with the New York Philharmonic Orchestra conducted by Leonard Bernstein, who became his lifelong friend. Other conductors with whom he has played include Carlo Maria Giulini and Rudolf Kempe. He founded the Schleswig-Holstein Music Festival in 1986 and became a UNHCR Goodwill Ambassador in 1989, a post from which he has since retired. He also founded the Philharmonia of the Nations in 1995.