 Robert Mackay was president of Herald Publishing Company, vice-president of the Bell Telephone Company of Canada, and sat on the board of directors of Canadian Pacific Railway, Royal Trust Company, Bank of Montreal, Montreal Light, Heat & Power Company, Dominion Textile Company, Limited, and others.A member of the Liberal Party of Canada, he twice ran unsuccessfully for the House of Commons of Canada as the Liberal Party of Canada candidate for the St. Antoine, Quebec riding in the 1896 and 1900 Canadian federal elections.In 1901, he was appointed by Prime Minister Laurier to the Senate of Canada where he served until his death on Christmas Day in 1916.