The house and surrounding land remained in Williams family ownership from 1920 until 1967. From the time her father died in 1933, Mary Williams lived at the house with her mother, until her mother died in 1950. In 1966, Mary and her brother Patrick agreed to sell the house and garden to the Northland Harbour Board. The sale was subject to the agreement that Mary could have life tenancy of the property and that it would be preserved as open space in perpetuity. Until her death in 1993, Mary fiercely resisted commercial proposals that might jeopardise the property's historic status or limit public access to the open space reserve.
Following Mary's death the Bay of Islands Mission Heritage Trust, a registered charitable trust acting on behalf of the Far North District Council, commissioned a conservation plan, (the Salmond Report), as a means of identifying appropriates uses for the site and buildings. As an outcome Williams House was registered in 1996 with the New Zealand Historic Places Trust, under the Historic Places Act 1993, as a Category 2 Historic Place, (as a place of "historical or cultural heritage significance or value"). Follow this link for the entry in the NZHPT register.
Restoration of the house and grounds took some time. After Mary's death, debate raged for years over what to do with the property. Meantime vandals had defaced the empty building to the extent that some locals wanted it to be demolished. However, the tenacity of Mary Williams in maintaining the property as open space reserve meant that finally the Far North District Council commissioned restoration work.
In 2003, the Paihia Library was finally moved out of a back room in the Paihia War Memorial Hall and relocated on the ground floor of Williams House. The library's reference section on the history of the Bay of Islands is regularly extended and the Friends of Williams House continue to fund-raise for further enhancements to the property.
As part of the Friends of Williams House involvement, a History Trail has been formed through the gardens. The trail will allow visitors to identify the items that are considered of historical importance to Williams House and Paihia.