Born on July 2, 1914 in Cleveland, Ohio, he studied at the Eastman School of Music at the University of Rochester, New York as a young man. In 1952 he founded the now famous Eastman Wind Ensemble, the world's first such musical group, at his alma mater. His recordings as conductor of the Eastman Wind Ensemble won worldwide renown for their superb musical interpretations and the high technical level of the sound, in which he took great personal interest.

Maestro Fennell was appointed Principal Conductor of Tokyo Kosei Wind Orchestra (TKWO) in January, 1984. The following year he received the Star of the Order from the John Philip Sousa Memorial Foundation, and in 1994 the Theodore Thomas Award from the Conductors Guild. In 1991 he and TKWO received the first Bandmasters Academic Society of Japan Award for their outstanding performances. Maestro Fennell served as its Conductor Laureate from 1996 until his death in December 7, 2004. He was ninety years old.