After belonging to the Byzantine Empire, in the Middle Ages it became part of the Giudicato of Cagliari. Several giudici (from Latin iudice, literally "judge") established their residences in Decimomannu. After the fall of the Giudicato of Cagliari, Decimomannu belonged to the Della Gherardesca family from Pisa. The Battle of Lucocisterna, fought between the Pisane and the Aragonese, took place here in 1324, and after it the town belonged to the latter (with the exception of a short period under the Giudicato of Arborea in 1353-1355). Decimomannu was part of the Kingdom of Sardinia in the Spanish Empire until 1708, when, together with the whole Sardinia, came under Piedmontese rule in 1720.
