 Williams was promoted captain on 5 January 1916. He was appointed a flight commander in No. 1 Squadron Australian Flying Corps (AFC), which was initially numbered 67 Squadron Royal Flying Corps by the British. The unit departed Australia in March 1916 without any aircraft; after arriving in Egypt it received B.E.2 fighters, a type deficient in speed and manoeuvrability, and which lacked forward-firing machine guns. Williams wrote that in combat with the German Fokkers, "our fighting in the air was of short duration but could mean a quick end", and that when it came to bombing, he and his fellow pilots "depended mainly on luck". He further quoted a truism in the Flying Corps that " if a new pilot got through his first three days without being shot down he was lucky; if he got through three weeks he was doing well and if he got through three months he was set. Williams and the other Australians were initially involved in isolated tasks around the Suez Canal, attached to various Royal Flying Corps (RFC) units. No. 1 Squadron began to operate concertedly in December 1916, supporting the Allied advance on Palestine. Williams completed his RFC attachment in February 1917.
