[9] In 1914 Harley volunteered to assist the war effort by serving as a nurse with the Scottish Women's Hospitals for Foreign Service (SWH) in France, where she was awarded the Croix de Guerre.

In June 1916 she established a motorised ambulance unit attached to the Royal Serbian Army in Macedonia that operated near the front line, often at night, despite district orders to the contrary.

In December 1916 she left the Scottish Women's Hospitals service to join an independent ambulance unit serving the civilian population of Monastir, Serbia (now in the Republic of North Macedonia).

[10] Her gravestone, a large white stone cross in Salonika (Lembet Road) Military Cemetery, reads "On your tomb instead of flowers the gratitude of the Serbs shall blossom there".

[13] A memorial fund was raised by the Women's' Citizens Association by 1924, which partly endowed a cot in the Royal Salop Infirmary in Shrewsbury and partly was invested in Conversion Stock to fund the annual award of the Sister Harley Memorial medals in gold and silver to two student nurses who scored the highest marks in exams; this was awarded during the lifetime of the hospital which closed in 1977.