Fred Shapiro and Hugo provided evidence from earlier dates in 1919.
The term demobilization, referring to the release of troops from military service at the end of a war, has been in use since the 19th century, but the abbreviated form demob seems to have been used only since the end of the First World War. The first quotations we have for the noun and verb respectively are:
Cambridge graduate, age 38, married elig. for demob., now home, can invest up to £500 in good class Practice or Partnership.
1919 Brit. Med. Jrnl. 28 June, p. 38/3
He’s demobbed, and has gone into the City. Horribly rich already, and will now, of course, make another pile.
1919 Mary Augusta Ward Cousin Philip iii. p. 52
Is there any earlier evidence for demob from the end of the war, when troops were first demobilized?
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To commemorate the centenary of the start of the First World War (1914–18), the OED is revising a set of vocabulary related to or coined during the war. Part of the revision process involves searching for earlier or additional evidence, and for this we need your help. Our first quotations are often from newspapers and magazines, and we know that there may well be earlier evidence in less-easily-accessible sources such as letters, diaries, and government records, many of which are now being made available in digital form for the first time.
Posted by OED_Editor on 29 January 2014 8.44