John Marquis Cahill received his early horn instruction from members of the Detroit Symphony and at the Interlochen Arts Academy where he was first horn and a concerto soloist. His formal training was at the Juilliard School where he earned B.M. and M.S. degrees studying with James Chambers. Additional studies were with Phillip Farkas, Joe Singer, Morris Secon, and John Barrows.

During his student years he was an active professional in New York as extra horn with the N.Y. Philharmonic and the N.Y.C. Ballet Orchestra, playing in Broadway shows, Radio City Music Hall Orchestra, Orchestra da Camera, and many chamber music ensembles. He was first horn in the Aspen Chamber Symphony and was a member of the New Jersey Symphony while earning his masters degree.

Upon graduating he joined the Toronto Symphony and began teaching at the University of Toronto Faculty of Music, where he presented many solo recitals as well as returning to New York City for a solo recital.
A long-time resident of the Caribbean, he has been an instructor at the University of the Virgin Islands as well as public and private schools. In addition to organizing choirs, chamber groups, and recitals, he presented solo horn recitals in New York, Detroit, and the Virgin Islands.

Currently he is active as a musician and teacher in Haiti where he has played first horn in the St. Trinite Philharmonic, appeared as horn soloist with the orchestra, and organized brass ensembles in three Haitian cities.

Most recently he has been invited by the Santiago, Cuba Symphony to organize a brass ensemble concert with the orchestra musicians next season.