An 1855 emigrant to Montreal, Canada East from his birthplace in Caithness, Mackay got his start working at the Henry Morgan & Company department store.

Highly successful in business, he became a close business associate of powerful Montreal entrepreneurs: Rodolphe Forget, stockbroker; Herbert S. Holt, President of the Royal Bank of Canada; and Vincent Meredith, President of the Bank of Montreal.

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.

Robert Mackay is interred with his wife Jane in the Mount Royal Cemetery in Montreal.

His daughter, Cairine Mackay Wilson, became the first woman appointed to the Canadian Senate.