Shackleton later recalled with gratitude how Greenstreet and his hunting partner, Alexander Macklin, had killed and brought in a Weddell seal weighing 800 pounds.

He accompanied his captain, Frank Worsley, in the Dudley Docker during the eight-day ordeal that marked the progress of the open-boat flotilla to a new and more secure campsite on Elephant Island in the archipelago of the South Shetlands.

Worsley, whose life would depend upon the success of this work,[12] describes it as follows: Frozen like a board and caked with ice, the canvas was sewn, in painful circumstances, by two cheery optimists – Greenstreet, Chief Officer of the Endurance, and Bakewell, a Canadian [sic] AB.

The only way they could do it was by holding the frozen canvas in the blubber fire till it thawed, often burning their fingers, while the oily smoke got in their eyes and noses, half-blinding and choking them.

All the time, while repeating the unpleasant task of thawing a length, and sewing it, 'Horace' [Greenstreet] was irrepressibly cracking his sailor jokes and Bakewell replying.

After the end of the expedition and with World War I continuing, Greenstreet was commissioned as a second lieutenant in the Inland Water Transport arm of the Royal Engineers in 1917,[14] serving in supply barges on the Tigris in British-occupied Mesopotamia.

"[17] On Elephant Island Worsley's last sight of Greenstreet prior to the Southern Ocean trip was his former First Officer, "cheerfully profane as ever", helping to bag stones to be loaded onto the James Caird as ballast.