Foster enlisted in the Australian Imperial Force (AIF) in 1914 and served with the 3rd Light Horse Regiment. He spent three months in Gallipoli, later serving with the regiment in Egypt before being sent home to Australia due to illness. In 1917, he became the inaugural state secretary of the Returned Sailors and Soldiers Imperial League of Australia (RSSILA). He was also the state secretary of the Reinforcements Referendum Council, formed to support the "Yes" vote in the 1917 conscription plebiscite.
