The Biddeford/Saco Mills Historic District is a cohesive collection of historic manufacturing buildings situated on roughly 38 acres of land flanking the Saco River. There are 42 contributing resources within the district limits, each associated with one of four distinct manufacturers: the York Manufacturing Company in Saco; the Saco Water Power Company; Laconia Company; and Pepperell Manufacturing Company in Biddeford. The district is significant for its association with the development of the Biddeford and Saco region from a remote seventeenth-century maritime settlement to a major industrial center. Buildings that make up the district are representative of the industrial development that fueled growth and expansion of Biddeford and Saco in the nineteenth century. It is also important as a cohesive collection of well-preserved industrial building types that represent the development of industrial architecture during the nineteenth and early twentieth centuries. The district attains further significance for its association with the Waltham system of textile manufacturing, which was utilized in major nineteenth century industrial communities throughout New England. The buildings were constructed following the slow-burning construction methods particular to the Waltham system and promulgated by the associated factory mutual fire insurance companies in the later nineteenth and early twentieth centuries. The district was listed in the National Register of Historic Places for its industrial and architectural significance.