They arrived on the Nimrod at Cape Royds in Antarctica in February 1908, and set up camp in a small expedition hut that would house the 15-man party through the winter.

In March, along with Douglas Mawson and Edgeworth David, Mackay made up the party who undertook the first ascent of Mount Erebus.

They still faced a 700-kilometre (435 mi) return journey and established a depot to enable them to transfer their load to one overladen sled and to remove the need to relay.

That day the party reached the coast line with perfect timing as within 24 hours they were collected by the Nimrod for the return trip to Cape Royds.

Mackay was also the ship's doctor on the ill-fated Karluk expedition in 1913 led by Vilhjalmur Stefansson to explore the regions west of Parry Archipelago for the Government of Canada.