Prior to entering electoral politics, in the late 1970s, McMillan was, successively, policy secretary to Robert Stanfield, Leader of the Official Opposition in the Canadian House of Commons; an executive officer of the Ontario Human Rights Commission; and Senior Research Associate of the national Commission on Canadian Studies, established by the Association of Universities and Colleges of Canada (AUCC).
These included the restoration of a unique pre-Confederation brick powder magazine—located in Brighton Compound in Charlottetown—the demolition of which the Canadian Army had begun until McMillan led a public campaign to stop it.
McMillan remained Environment minister until he was defeated in the 1988 general election due to opposition to the Canada–United States Free Trade Agreement among his constituents.
McMillan was awarded an honorary doctorate by Bridgewater State University in 1993 for—in the words of its president, Dr. Adrian Tinsley -- "significant accomplishment on behalf of Canada in support of strong, enduring Canadian-American cooperation and ties."
Among private sector and community services, McMillan has been chairman of the Book and Periodical Development Council of Canada and headed the Canadian Chamber of Commerce Task Force on the Environment and Economy.