On his eighth birthday, Mackay's father took him on a flax run between Montrose, Angus and Archangel in Russia; thereafter he never "missed an opportunity to converse with captains in port".

In 1871, he secured employment with the shipbrokers and agents Gellatly, Hankey and Sewell, who were involved with the newly founded British-India Steam Navigation Company (BI).

The opening of the Suez Canal in 1869, which shortened the journey from London to Bombay by 4,000 miles (6,400 km), created a surge in trade between Europe and India.

[6] As a result, BI's Calcutta agents, Mackinnon Mackenzie & Co., asked their London based counterpart, Gray, Dawes & Co., for a new shipping assistant to handle the increased workload.

[7] With the development of inland transport throughout the sub-continent, export opportunities in indigo, coal, tea, jute and wrought iron abounded and saw Mackinnon Mackenzie & Co. enjoy a steady increase in business.

By late July 1902 the negotiations were reported as practically concluded when the British team arrived back in Shanghai,[12] and the treaty was signed by Mackay and Sheng Xuanhua as representatives of their governments on 5 September 1902.

[16] Mackay was largely responsible for solving India's currency problems and for the adoption of the Gold Standard, he was made a baron by King George V for his services to industry and nation in 1911.

In 1924 he was made a Knight Grand Commander of the Order of the Star of India (GCSI) and created Viscount Inchcape, of Strathnaver in the County of Sutherland.