While a graduate student, Leonhard concurrently worked as an instructor and assistant professor of music education at Teachers College.
He had previously served in the United States Army during World War II as a field artillery officer.
In addition to Bowman, many important philosophers of music education can trace their lineage back to Leonhard.
Mark and Gary acknowledge the importance of Leonhard's contributions to the history of music education in the United States.
[10] David J. Elliott puts Leonhard in the company of Peter W. Dykema, Karl Gehrkens, James Mursell, Lilla Belle Pitts, Harry Broudy, Abraham Schwadron, Bennett Reimer, and Keith Swanwick as leading thinkers in music education.