To judge from the way his name was cited in later sources, Johannes de Garlandia was apparently a magister in the University of Paris, whose surname was derived from the clos de Garlande, an area of the Left Bank where many masters and students lived. Hieronymus de Moravia, the late 13th-century compiler who transmitted the revised version of the treatise on mensural music, also called him Johannes Gallicus, indicating that he was French. Although several scholars (notably Waite) have sought to identify him with the English poet and grammarian Johannes de Garlandia (c1190–c1272), who taught in Paris in the second quarter of the 13th century, almost all recent scholarship (beginning with Reimer and Rasch) has rejected this idea. It now seems probable that the theorist was not active before the 1270s, as postulated by Huglo (1986) and others.
Although the earliest source of De plana musica and De mensurabili musica is a Parisian manuscript copied about 1260 (I-Rvat lat.5325), no author is named in this or any other source of the original versions of the treatises, and the earliest citations of De mensurabili musica, by Anonymus 4 (CoussemakerS, i) and the Sowa or St Emmeram anonymus, are also nameless. The name of Johannes de Garlandia is first associated with the treatises towards the end of the 13th century by Hieronymus de Moravia, who incorporated into his own compilation a revised version of De mensurabili musica; in consequence Pinegar has suggested very persuasively that Johannes de Garlandia was not the original author or compiler of either treatise but rather the person who revised the text for Hieronymus. Whitcomb has made a circumstantial case for identifying him with Jehan de Garlandia, a bookseller (librarius) on the rue des Parchemeniers, who appears in Parisian tax records and other documents between 1296 and 1319. Johannes de Garlandia himself (as distinct from the anonymous author of the original treatises) would thus have been contemporaneous with the other writers or compilers of substantial treatises on mensural music in the latter decades of the 13th century – Lambertus, Anonymus 4, the St Emmeram anonymus, Franco of Cologne and Hieronymus de Moravia – as had long been suspected by Reckow.