The life of St. Guillermo is the subject of somewhat contrasting and divergent opinions; this because the historical documentation is scarse and the fact that, with the passage of time, records of the lives of various personages got mixed up. Guillermo IX, count of Pattavia and duke of Aquitania, who died as a pilgrim in Santiago de Compostela; Gulliermo of Tolosa who led a monastic life; Guillermo the hermit who lived and died as a hermit at Malavalle.

These historic personages are not similar only in name, but also because of other factors in their lives: they were all noble and had taken consecrated life as monks or hermits. According to a manuscript of the late XVI century (Vita del Beato Guglielmo C. Duca d’Aquitania, et conte di Pittavia, Restauratore  dell’Antica religione Agostiniana autore Tebaldo Francese vescovo cantauriense, et suo compagno tradotta dal latino al volgare per me Fra Michele Angelo Carli da Scarlino del detto ordine Agostiniano) Guillermo, of noble birth, in the year of the 1130 schism after the death of Pope Onorjo II, associated himself with the anti-pope Anakleto. St. Bernard of Chiaravalle tried his best to get him to respect and show obedience again to the true successor of Peter, something that he managed to achieve after many years.

In making this conversion, Guillermo – as a sign of penance – placed against his body a metal breast plate (known as a giacco) that got stuck to his skin and remained on him throughout his lifetime.  On going to the Pope, as a form of penance, he was sent on a pilgrimage to the Holy Land. Back from Jerusalem, he went on a pilgrimage to Santiago de Compostella where he feigned death so as to leave behind him, symbolically but not in a definite way, his previous life. Through this choice of life, he retired to a place close to Pisa, in Livallia, but soon after he had to move away to Maremma where he again started the life of a hermit in a room which he built himself. He died what was widely considered to be a saintly death in the arms of one of the disciples who were around him, named Alberto, on 10 February 1157, close to Castiglione della Pescaia.

His tomb immediately started attracting various pilgrims from everywhere. Pope Alexander III approved his cult which, later, was confirmed by Pope Innocent III. The disciple closest to him wrote The Constitution or Rules of Guillermo inspired by what was said about Guillermo and his exemplary life. A number of disciples formed the congregation of Guillermo that spread to various parts of Italy and abroad: France, Belgium, Bohemia and Hungary. When in 1256 the Augustinian Order that was established in 1244 grew by the association of a number of other congregations, amongst these were the Gullielmiti. However, these did not have a firm desire to make this association imposed from the top and, after a few months they managed to obtain a papal bull so as to be dissolved from the Augustinian Order.

But a number of convents away from the Alps where, apparently, this bull remained unknown, stayed within the Augustinian Order. For this reason, the Augustinian Order, for a long time, has celebrated the memory of St. Giullermo of Malavalle as from 1290. The life of a hermit lived by St. Guillermo reminds us of the highest place that we should give God in our life. At the same time the life of penance which he lived teaches us the road to a constant conversion and woe to him who says he has reached that, because, as St. Augustine says, from that moment he will start moving backwards.