Marianus fell a victim to the plague while engaged in administering the last sacraments to the inhabitants of his native city.
His most noted work, Fasciculus Chronicarum, is a history of the Franciscan Order from the beginning up to the year 1486.
Though it was written three centuries after the death of Francis, it is not necessarily untrustworthy, for he had access to original sources now lost, of which some fragments have been passed on through him.
Besides the Fasciculus Chronicarum, he is the author of a Catalogus seu brevis historia feminarum ordinis Sanctæ Claræ which contains biographical sketches of more than 150 illustrious women of the Second Order of St. Francis.
Among his other writings may be mentioned: Marianus's Historia Translationis Habitus Sancti Francisci a Monte Acuto ad Florentiam has been translated into Italian and published by Roberto Razzoli in his monograph, La Chiesa d'Ognissanti in Firenze, Studi storicocritici (Florence, 1898).