He was probably French, probably lived and worked in Paris at the Notre-Dame Cathedral and was the earliest member of the Notre Dame school of polyphony and the ars antiqua style who is known by name.
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.
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."
According to Anonymous IV, Léonin's work was improved and expanded by the later composer Pérotin.
The musicologist Craig M. Wright believes that Léonin may have been the same person as a contemporaneous Parisian poet, Leonius, after whom Leonine verse may have been named.