Antonio Piccolomini was the son of Andrea Piccolomini, of the lords of Modanella, in the same branch of the Piccolomini family as Pope Pius II.[3]
He was ordained a priest in the Order of Saint Benedict.[2] He was appointed abbot of the Camaldolese monastery of S. Vigilio in Siena, which was under the patronage of the Piccolomini family.[4] He was succeeded by Francesco Todeschini-Piccolomini (Pope Pius III), who was named Abbot Commendatory in 1458.[5]
On 18 September 1458, Antonio was appointed Bishop of Siena by the newly-elected Pope Pius II, who had just vacated the bishopric of Siena.[6] Pius II granted him the privilege of being consecrated a bishop by any bishop he chose who was in communion with Rome.[7] As bishop-elect, he took possession of his new diocese on 28 September 1458.[8] On 16 April 1459, he was consecrated bishop, though the names of the consecrators remain unknown.[2]
On 18 April 1459, at the urging and with the cooperation of Pius II, who lent Bishop Antonio the services of Agapito Cincio de' Rusticii, a Referendary and Auditor Causarum in the papal Court, there was issued a new Constitution for the diocese, consisting of fifty-six articles.[9]
On 23 April 1459, Pope Pius II elevated the diocese of Siena, of which he had recently been the bishop, to the rank of Metropolitan Archbishop of Siena.[10]
Antonio Piccolomini served as Archbishop of Siena a total of seventeen months, until his death on 8 November 1459.[1][2] He had gone to Monte Amiata to the baths of S. Filippo, for health reasons, and died at the castle of Crevole, which belonged to the bishops of Siena.[11] He was succeeded on 6 February 1460 by Francesco Todeschini-Piccolomini.[12]
