In March 1943 Long was appointed general editor of the Australia in the War of 1939–1945, a 22-volume official history of Australia's involvement in the Second World War, on the recommendation of C.E.W. Bean, the editor of the Official History of Australia in the War of 1914–1918.
Based at the Australian War Memorial in Canberra, he spent the remainder of the war planning the series and visiting forward areas to interview members of the Australian military.

After the war Long played a key role in the official history project.
As well as providing guidance to the other authors, he wrote three of the volumes in the series (To Benghazi (published 1952), Greece, Crete and Syria (1953) and The Final Campaigns (1963).
He retired as general editor in 1963 as the project was nearing completion and he did not believe that a full-time editor was required.
Long's books were well received by reviewers and his close involvement with the other authors gave the series a unity of purpose and method.
Long was appointed an Officer of the Order of the British Empire in 1953 for his services as editor of the official history.
In 1956 he was awarded the Greek Gold Cross of the Royal Order of the Phoenix.

Long continued to write after his retirement from the official history project.
He was a research fellow with the Australian Dictionary of Biography, was part of the team which produced the Australian Government's Style Guide and contributed over 90 articles to The Canberra Times.
He also wrote two further military history books, MacArthur as Military Commander (published in 1969) and The Six Years War (1973), which was a concise summary of Australia's involvement in the Second World War.
The Six Years War was written well before it was published, but its publication was delayed while the final volumes in the official history series were completed.

Long died of lung cancer on 10 October 1968 at his home in Deakin, Australian Capital Territory, and was cremated.
In March 1943 Long was appointed general editor of the Australia in the War of 1939–1945, a 22-volume official history of Australia's involvement in the Second World War, on the recommendation of C.E.W. Bean, the editor of the Official History of Australia in the War of 1914–1918. Based at the Australian War Memorial in Canberra, he spent the remainder of the war planning the series and visiting forward areas to interview members of the Australian military.
After the war Long played a key role in the official history project. As well as providing guidance to the other authors, he wrote three of the volumes in the series (To Benghazi (published 1952), Greece, Crete and Syria (1953) and The Final Campaigns (1963). He retired as general editor in 1963 as the project was nearing completion and he did not believe that a full-time editor was required. Long's books were well received by reviewers and his close involvement with the other authors gave the series a unity of purpose and method. Long was appointed an Officer of the Order of the British Empire in 1953 for his services as editor of the official history. In 1956 he was awarded the Greek Gold Cross of the Royal Order of the Phoenix.
Long continued to write after his retirement from the official history project. He was a research fellow with the Australian Dictionary of Biography, was part of the team which produced the Australian Government's Style Guide and contributed over 90 articles to The Canberra Times. He also wrote two further military history books, MacArthur as Military Commander (published in 1969) and The Six Years War (1973), which was a concise summary of Australia's involvement in the Second World War. The Six Years War was written well before it was published, but its publication was delayed while the final volumes in the official history series were completed.
Long died of lung cancer on 10 October 1968 at his home in Deakin, Australian Capital Territory, and was cremated.
Gavin Long