Promoted to the honorary rank of captain in early 1918, he served in a secret mission in Caucasus under General Lionel Dunsterville.
Appointed on 8 March 1922, Williams was promoted to Government Anthropologist when Strong retired in 1928, and kept the position until the demise of the Papuan administration in 1942.
One of the few anthropologists of his time able to spend two continuous decades in the same location without having to regularly return to a metropolitan university or institution, he performed during those twenty years heavy field work, and published many books and articles, both monographic and general.
When World War II expanded to the Southern Pacific in December 1941, Williams came back to Australia and enlisted as a lieutenant to serve with military intelligence.
In early 1943, Williams was sent back to Papua to serve as a liaison officer with the Australian New Guinea Administrative Unit.