He is best known for the 1930 book 1066 and All That, a tongue-in-cheek guide to "all the history you can remember," which he wrote together with R. J. Yeatman.
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On leaving Oriel, Sellar worked as a schoolmaster at his old school Fettes, leaving in 1928 when he moved to Great Marlow in Buckinghamshire in the hope of becoming a full-time writer.
This style was pioneered by the British humorists W. C. Sellar and R. J. Yeatman with their parody of British history '1066 and All That' in the 1930s.