Koster grew up in New York and studied at [url=https://discogs.com/label/713945]St. Thomas Church[/url]'s Choir School between 1953 and 1963. John then relocated to Massachusetts and attended [url=https://discogs.com/label/1705359]Mount Hermon School[/url] (1963–67). He graduated "cum laude" from Harvard University with an A.B. degree in Music in 1971 after studying with Gustav Leonhardt and taking private organ lessons with Marian Ruhl Metson. In 1975, Koster became a Musical Instrument Conservation and Research Consultant at the Museum Of Fine Arts Boston, where John worked for sixteen years. In January 1988, he attended a Wood Identification Workshop at the University Of Massachusetts. In 1991, he joined the Metropolitan Museum Of Art's Department of Musical Instruments as "Andrew W. Mellon Senior Fellow," conducting research on the history and technology of free-reed instruments in Asia and Europe from XVII and early-XVIII centuries during his one-year tenure.