First historical mentions of the "Castle of Monticiano" dates from 1171, when it was under the suzerainty of the bishop of Volterra. During the Medieval period the forests represented an important resource. Not only did they provide wood but also food supplies such as wild game and chestnuts. A system of agriculture quickly grew up around Monticiano, the principal product cultivated being wheat. In 1266, due to its citizens' participation to the Ghibellinedefeat at Benevento, it was occupied by the Sienese troops, who destroyed the castle walls. In 1554 it became part of the Grand Duchy of Tuscany. From 1629 to 1749 it was a fief of the Pannocchieschi family.
In 1860, its 723 inhabitants unanimously agreed to join the Kingdom of Italy. During World War II, partisans from the Siena zone started their first organizational operations in Monticiano, where a resistance brigade, named after Spartaco Lavagnini, also operated. The night time battle between the partisans and the Germans between the 3rd and 4 June 1944 which took place in the town's central piazza is particularly remembered.
