The son of Alexander and Agnes (Stevenson), Charles Leonard T. MacKay was born on 7 September 1879, in New Glasgow, Prince Edward Island. Educated in common schools and the Charlottetown Business College, Charles entered business life in 1897, working as a bookkeeper with the Bruce Stewart & Company Limited of Charlottetown in 1897. By 1910 he had been appointed secretary-treasurer of the company. Charles was a Major and commanded the No.1 Battery in the Canadian Heavy Artillery at the outbreak of World War 1. McKay detailed for duty at the Halifax Fortress, commanding the Ives Point Battery and later as fire commander of the fortress. In 1919 he was relieved of duty and returned to Charlottetown to work for Bruce Stewart.

Charles married Maude Billingsley of Charlottetown, daughter of Richard Billingsley, Sacramento, California, in 1905. They had one son and one daughter. MacKay was a Director of the Island Fertilizer Company Limited, owner of the Lawndale Fox Company, member of the Board of Governors of the Church of Christ in Toronto, Ontario, President of the Island Missionary Society of the Central Christian Church in Charlottetown, and member of the Charlottetown Board of Trade.