Mongrassano already existed in the 12th century as a hamlet of San Marco named Mons Crasanus, Mocrasani, Montegrassano or Magrosani. It was originally divided in two hamlets: Serra di Leo and Mongrassano. Like many other Albanian-origin settlements the urban structure was based on the gijtonia, in other words, circular streets. Either in 1459 or early in the 16th century, another group of Albanian political refugees came and settled in Mongrassano.
On 20 July 1459 the Prince of Bisignano gave the civil jurisdiction of the city to the Bishop of San Marco. In 1642 the Gaetani acquired the territory; in 1688 they sold the property to the Marquesses of Fuscaldo, who held it until the end of the feudal system in 1806. In 1807 Mongrassano became state owned and in 1811 it joined with Serra di Leo. In 1816 it was elevated to the level of a comune, with Serra di Leo as its frazione.