Sir John Johnson named Williamstown after his father, Sir William Johnson. It was one of his settlements for the Loyalists.
The site, on banks of the Raisin River (first named in French- Rivier Aux Raisin- river of the grapes for the abundance of wild grapes that once lined its shores), had been scouted out to be an excellent location to start a settlement. It was the right spot to build mills( grist mill and saw mill), the land was good and there was good timber. At this location Sir John was given a substantial grant of land for himself, which included the land on which the Village of Williamstown is located. On this property, sometime between 1784 and 1792, Sir John built, what is described as a one and a half story, five-bay log structure. He built a grist mill and a saw mill close by on the river in the same period. The mills were the economic centre of the community. The house was occupied by the overseer of the mill for at least part of Sir John’s ownership of the site. Before 1820, most likely 1818, the house and lands were purchased by Hugh McGillis, a retired fur trading partner. He continued to run the mills, and, in addition, developed the land agriculturally. Hugh McGillis was referred to as the Laird McGillis, and it is believed that at this time the house became known as the Manor House.By 1872 the last of two major additions had been made to the house. By this time Williamstown had grown with three churches and numerous businesses. The Manor House ceased being a family residence in 1956. The house was declared a place of national historic significance in 1961. From 1956 until 1971 the property was owned and occupied by the Sisters of the Sacred Heart. In 1971 the house was sold to Parks Canada and designated a national historic site. The house is currently occupied by the Sir John Johnson Manor House Committee.