All that is known about him comes from the writings of a later student at the cathedral known as Anonymous IV, an Englishman who left a treatise on theory and who mentions Léonin as the composer of the Magnus Liber, the "great book" of organum. Much of the Magnus Liber is devoted to clausulae—melismatic portions of Gregorian chant which were extracted into separate pieces, with the original note values greatly slowed down, and provided with a fast-moving upper part. Léonin was also probably the first composer to use the rhythmic modes, and possibly also to invent a notation for them (according to W.G. Waite, writing in 1954: "It was Léonin's incomparable achievement to introduce a rational system of rhythm into polyphonic music for the first time, and, equally important, to create a method of notation expressive of this rhythm.").
The Magnus Liber was intended for liturgical use. According to Anonymous IV, "Magister Leoninus (Léonin) was the finest composer of organum; he wrote the great book (Magnus Liber) for the gradual and antiphoner for the sacred service." All of the Magnus Liber is for two voices, although little is known about actual performance practice: the two voices were not necessarily soloists.
The musicologist Craig Wright believes that Léonin may have been the same person as a contemporaneous Parisian poet, Leonius, after whom Leonine verse may have been named. This would make Léonin's use of meter even more significant.