The origins of Casorate Primo are not clearly known. The first evidence of settlement in the current location of the town date back to the 2nd and 3rd centuries AD, during the ancient Roman dominion of the area (at which time the land was part of Cisalpine Gaul, due to being inhabited by Celtic peoples, such as the Insubres). Nonetheless, due to a lack of archaeological and historical evidence, it is possible that there were settlements on the territory from before this period. By the 5th-6th century, the village was fully Christianized.