The Civic Museum occupies the first floor of the Public Palace of Siena, one of the best known and most significant Gothic buildings in medieval Italy, built at the behest of the Government of the Nine (1287-1355). The works for the construction of the building, probably carried out starting from 1297 partly reusing pre-existing walls, initially involved the central body already completed in 1304 with the construction of the so-called Mappamondo room, seat of the meetings of the General Council of the Republic until 1343. Between 1307 and 1311 a first enlargement would have affected the area on the right towards Malborghetto with the construction of the so-called wing of the Nine.

According to what is attested by ancient chronicles around 1325, the extension to the left of the central body towards Salicotto would have begun with the construction of the wing of the Podestà, completed after 1330. The building of the building would have ended in 1348 with the construction of the travertine fortress of the Torre del Mangia, erected starting from 1325 in the corner of the street called Malcucinato and with the arrangement of the large loggia on the back in 1350. Built with the main function of hosting both the politics and the major administrative functions of the ancient Sienese state, only with the unification of Italy the palace became open to visitors in appointed days and, following the great success obtained by the "Exhibition of Ancient Sienese Art", set up inside in 1904, permanently open since 1907 relative to some rooms on the noble floor. Its official establishment can instead be traced back to 1909, the date of approval of a specific regulation. Over the years the Museum, having to coexist with the institutional and administrative functions still housed inside the Palazzo, has seen extreme variability both in terms of the extension of its spaces and in the collections on display (topographic, numismatic, pictorial). The most recent exhibition and museographic projects date back to 1985 and 1992 respectively.

The decoration of the building, which follows the various construction events hand in hand, still constitutes its main feature today. The rooms on the main floor, which represent the fulcrum of the Civic Museum, present in particular a dense concentration of frescoes and wall paintings belonging to the most important interpreters of the Sienese school of painting from the fourteenth to the nineteenth century. Masterpieces such as the Majesty (1315-1321) or the Guidoriccio da Fogliano at the siege of Montemassi (1330) by Simone Martini, the Presa of the castle of Giuncarico (1314 ca.) attributed to Duccio di Buonsigna, the Allegories and the Effects of the Good and bad government (1338-1339), the pictorial cycle with the public Virtues and episodes of the Greek and Roman heroes (1529-1535), made by Domenico Beccafumi on the ceiling of the Consistory room, are unanimously recognized as art virtues not only from Siena but worldwide. Beyond the imposing decorative apparatus that makes it a context of unparalleled beauty, the museum also preserves some important collections that have stratified over time and are now only partially exhibited, which attest to the complex and articulated history of its creation, including in addition to the works directly commissioned for the palace, the goldsmiths, the valuable numismatic collections and an important selection of sculptures, paintings on wood and canvas of the Sienese and non-Sienese school stand out.