Sarah de Mestre was born on February 8, 1877 to parents Andre and Augustus de Mestre.
At the outbreak of WWI Sarah joined the first ship to be fitted out as an Australian hospital ship, the Grantala, which accompanied the Australian Naval and Military Expedition Force to German colonies in the Pacific.
The Grantala sailed from Sydney on August 6, 1914 and arrived at Rabaul on September 13. Within the hour, wounded soldiers were brought aboard.
On April 26, 1915 Sarah enlisted in the Australian Army Nursing Service and joined the third Australian General Hospital staff on March 27, 1916.
She worked in Lemnos, Egypt and then France, where she became second in charge of the No.2 Australian General Hospital at Abbeville on the Western Front.