 San Giorgio was a member until 1535 of the municipality of Legnano, from which it was divided after the administrative reform of Charles V, Holy Roman Emperor.After the Italian unification, following the enactment of the royal decree N°941, on 23 October 1862, the municipality took the name "San Giorgio su Legnano" to distinguish it from other municipalities of the same name.In Prehistory the "sangiorgese" territory was covered by moorland.