Treitler studied at the University of Chicago under Grosvenor Cooper, achieving the BA in 1950 and the MA in 1957. He received an MFA from Princeton University in 1960 and a Ph.D. in 1967; there he studied under Oliver Strunk, Arthur Mendel, and Roger Sessions. From 1961 to 1965 he taught at the University of Chicago, and following this at Brandeis University and SUNY Stony Brook.
Treitler's major work is in Medieval and Renaissance music, particularly in Gregorian chant and the earliest polyphony. He also published a series of essays exploring historiography in music history, which were collected, with other works on music history and theory, in Music and the Historical Imagination. He revised Oliver Strunk's Source Readings in Music History in 1998.
- The Aquitanian Repertories of Sacred Monody in the Eleventh and Twelfth Centuries (dissertation, Princeton U., 1967)
- Music and the Historical Imagination. (Cambridge, MA, 1989) [collection of essays]
- Source Readings in Music History. New York, 1998 (orig. ed. O. Strunk, pub. 1950)
- With Voice and Pen: Coming to Know Medieval Song and How it Was Made. (Oxford, 2003)
On the rise of Western plainchant and notation
- Homer and Gregory: The Transmission of Epic Poetry and Plainchant. The Musical Quarterly Vol. 60, No. 3 (Jul., 1974), pp. 333–372
- "Centonate" Chant: "Übles Flickwerk" or "E pluribus unus?". Journal of the American Musicological Society Vol. 28, No. 1 (Spring, 1975), pp. 1–23
- The Early History of Music Writing in the West. Journal of the American Musicological Society Vol. 35, No. 2 (Summer, 1982), pp. 237–279
- Reading and Singing: On the Genesis of Occidental Music-Writing. Early Music History Vol. 4 (1984), pp. 135–208
- The "Unwritten" and "Written Transmission" of Medieval Chant and the Start-Up of Musical Notation. The Journal of Musicology Vol. 10, No. 2 (Spring, 1992), pp. 131–191
On historiography and musical analysis
- The Present as History. Perspectives of New Music Vol. 7, No. 2 (Spring, 1969), pp. 1–58
- History, Criticism, and Beethoven's Ninth Symphony. 19th-Century Music Vol. 3, No. 3 (Mar., 1980), pp. 193–210
- "To Worship That Celestial Sound": Motives for Analysis. The Journal of Musicology Vol. 1, No. 2 (Apr., 1982), pp. 153–170
- Morgan/Sparshott, "Leo Treitler". The New Grove Dictionary of Music and Musicians online.