MacMillan also served as Chairman of the Greenock Harbour Trust and of the Firth of Clyde Drydock at the time of its establishment. He was appointed the first Chairman of the Cumbernauld Development Corporation, responsible for building a "new town" between Glasgow and Stirling. From 1955 to 1980, he also chaired the Executive Committee of Erskine Hospital which had been created as a hospital and care home for ex-service men and women in the First World War. Other voluntary work involved him as Chairman of the Scottish Police Dependants' Fund and the City of Glasgow Council of Social Service. He was awarded an Honorary Doctorate in Law (LLD) by Glasgow University in 1969. MacMillan died in a car accident on 21 January 1986, just two weeks after his eighty-ninth birthday. He is buried against the north wall in the highly overgrown section of Newington Cemetery in Edinburgh but a path has been created to his grave.