He was born in Paris in 1135 and died in approximately 1201. He received his training at the Notre Dame Cathedral schools and later became a priest at Notre Dame.

He is especially known for his development and original writing of polyphonic music (music that has two or more lines occurring at the same time).

Leonin's compositions were based on the Gregorian chants. The original chant served as the foundation and a second voice, called the descant, was added to the original chant.

This style came to be known as organum (example). One of Leonin's major contributions to music was a collection of organum with two-part settings of portions of the mass known as the Magnus Liber Organi.