During World War I he served in the Durham Light Infantry, being awarded the Military Cross and the Croix de Guerre.

He was appointed Crown Counsel (a public prosecutor) in the colonial government of Nigeria in 1924.

On 28 August Fitzgerald issued his report, which proposed dividing the city into autonomous Jewish and Arab boroughs.

Towards the end of the mandate he attempted to find a judicial post in England, but was unsuccessful.

The anonymous Irish Times obituarist observed of Sir William's service in Palestine that-- ... Sir William is said to have remained "rigidly unpolitical and a man of moderation," whose "evident Irishness was perhaps a help" and who wrote extensively about the importance of Jerusalem's spiritual and cultural legacy.