Priestley had completed his second year of studies when he enlisted as a geologist for Shackleton's "Nimrod" Expedition (1907–09) to Antarctica. There he worked closely with renowned geologists (Sir) Edgeworth David and Douglas Mawson, also members of the expedition. Priestley collected mineral and lichen samples from the region including islands in the Ross Sea, the North face of the Mount Erebus volcano, and mountains near the Ferrar Glacier. He was part of the advance team that laid the food and fuel depots for Shackleton's nearly successful attempt to be the first to reach the South Pole in 1909. In a November 1908 expedition, due to a lack of space in a tent, Priestley spent three days of a blizzard sleeping outside in his sleeping bag. As the blizzard raged, he slowly slipped down the glacier and nearly fell off its end to his death. On his return from the expedition, he spent four months in England before returning to Sydney, Australia, to work with Edgeworth David on the geological report, eventually published in 1914.