The Architectural Conservancy of Ontario (ACO) is non-profit, charitable organization set up in 1933 to preserve the historic built environment and nature landscape in Ontario.

A committee was set up and a brochure issued in Oct. 1932 for the organization of a "Society for the Preservation of Early Architecture and Places of Natural Beauty in Ontario." The Architectural Conservancy of Ontario was founded through the issuance of Letters Patent under the Ontario Companies Act on 1 Feb. 1933. The express purpose of the organization was to advocate the protection and conservation of Ontario's architectural and landscape heritage, to preserve buildings, structures and places in Ontario deemed to be of architectural merit, natural beauty or historic interest.

The main founder of ACO was Prof. Eric Ross Arthur. Other names prominent among the organization's first directors were: Vincent Massey, the architects Howard Dunnington Grubb, Alvan Sherlock Mathers, John MacIntosh Lyle, and Mackenzie Waters, the provincial historian Verschoyle Benson Blake, and the notable librarian William Stewart Wallace.

The ACO has been working to find economically viable uses for Ontario's historical assets, ensuring that the distinctive buildings of the past do not become parking lots or building sites for characterless new development.

The ACO developed a structure of many volunteers at both the local branch and provincial levels, with a central office, the ACO Council, an executive and an Advisory Board. In 2003 there were 11 branches throughout Ontario. Other branches have formed but ceased operations over the years. These branches develop and administer their own programs and fundraising events; work to preserve and restore local sites and structures; and publicize the need to maintain our architectural and landscape heritage.

The ACO publishes Acorn, The Journal of the Architectural Conservancy of Ontario, and holds an Annual General Meeting each November.