With the outbreak of World War II Downer was posted to the Australian Associated Press office in London. In 1940 Sidney enlisted in England with the Royal Air Force, and was posted as a publicity officer with the rank of Flight Lieutenant in 1941 to Singapore. He escaped with others to Malaya following the fall of Singapore but was captured in Java and interned in Formosa (now Taiwan), followed by Manchuria. He was repatriated to England at war's end, then returned to Adelaide late in 1946, where he resumed working with "The Advertiser" as a sports writer, but his marriage to Melbe was over, to be replaced by the widow Margaret Dutton, daughter of Victor Marra Newland of Adelaide's prominent Newland family. Marital happiness was not to last however, and a few years later they separated, and Sidney took up with Dorothy "Doss" Anderson, née Clampett, whom he married in 1964.