The most important town archives, fruit of a common production network and a general archive transmission, were gathered together in 1957 to form one single institute of conservation: the "Subsection of the Prato State Archives", founded in the same year and located at palazzo trecentesco that was once the home of Francesco di Marco Datini.
From 1963, it was called "State Archives Section", under the authority of the Florence State Archives, in accordance with the provisions governing the Italian Archive Administration and prescribing the presence of State Archives in each province (President's Decree n. 1409 of 30 September 1963). As per the decree of the Ministry of Cultural Heritage and Environmental Conservation of 24 May 1997, following the setting up of the Province of Prato, the Section has become State Archives.

The documentary patrimony preserved at the Prato State Archives is evidence of relevant features of the town history, starting from the institutional aspects of the local government and associated life (Municipality, welfare bodies and charities, ecclesiastical authorities, etc.) up to a private and family scale (archives of families and individual persons).

One of the most important funds is that of Comune, consisting of 7294 pieces included chronologically between 1237 and 1899. It preserves documents typical of the institutional activity of the medieval municipality, such as statutes, deliberations (called "diurnals" in ancient times), drafts, imposition of taxes, etc., and records of modern communities (chancellor's acts, correspondence with the central authority, etc.), as well as papers relating to the judicial magistracies (called rule of a podestà, praetorship and vicariate).

However, the fund also contains series showing unique characteristics such as stato civile, that is to say the registration - as per the copy of the baptismal register kept at the ecclesiastical authorities - of the births of citizens in Prato from 1482 (and of the deaths from 1557), together with the corresponding alphabetical indexes.