He came from a noble family who had directed him to an ecclesiastical career. When he was elected Bishop he was archdeacon of the Cathedral. He did not neglect the pastoral care of the city: he put an end, for example, to the dissension that divided the canons of the Pieve and those of the Cathedral, gave a decisive impulse to the development of the Hospital of Santa Maria del Ponte, approved the establishment of the Fraternity of Santa Mary of Mercy (later called the Fraternity of the Laity).
He took part in the Second Council of Lyons and welcomed to Arezzo, Pope Gregory X, tired and sick while he was returning to Rome from that Council. The Pope died in the city where the conclave was also held. A donation from the Pope determined the beginning of the construction of the "new" Cathedral: a building in Gothic style built inside the city walls, unlike the "Old Cathedral". He also built the current bishop's palace.
He was also a politician and leader, and he did not disdain the use of force. He ended a dispute with the monastery of Camaldoli by making the monks beat his soldiers by attracting the Pope's censorship.
He chose the Ghibelline part most likely to try to maintain the independence of Arezzo and not for real political conviction. Now in old age he led the Ghibelline army to the Battle of Campaldino where he was defeated and lost his life on Saturday 11 June 1289 for a shot of pike to the head.
His mortal remains, collected on the battlefield, were buried, together with those of his nephew, also dead in battle, in the nearby church of Certomondo, at the foot of the castle of Poppi and found there in 2008 and attributed to him through accurate scientific examinations (radiocarbon, DNA, anthropometric examination).
On 11 June 2008, seven hundred and nineteenth anniversary of the battle, at the end of an impressive ceremony of suffrage, celebrated in the Cathedral of Arezzo by the bishop Gualtiero Bassetti, the body of Guglielmino was finally given the extreme home in the cathedral that he had founded over 7 centuries before.