In World War I, Himmatsinhji received a temporary commission in the British Indian Army, and saw service in Mesopotamia. Two of his relatives also served overseas during the war – a cousin, Savaisinhji Devisinhji, was wounded in African campaign, while a brother, Dajirajsinhji, was killed in action in France in 1917. By the end of the war, Himmatsinhji had been promoted to captain. On 26 March 1919, Himmatsinhji received a substantive commission in the British Indian Army as a second lieutenant, with date from 21 September 1918. He was promoted to lieutenant on 21 September 1919, and to captain on 26 March 1924.