Legg was born on 31 August 1910 in London into a middle-class household. His father was a solicitor. Legg graduated from Cambridge with a degree in engineering. His first film was Varsity with the university's Film Society. This was followed by Cambridge, produced with some involvement from British Instructional Films.
Filmmaking career
After graduation, Legg worked for six months as an assistant to director Walter Creighton at Publicity Films, a commercial company. As part of the British Documentary Film Movement, Legg worked with John Grierson. His first film after being taken on by Grierson was The New Generation for Chesterfield Education Authority, said to "exemplify an attempt at the Russian technique."Joining the General Post Office Film Unit GPO Film Unit from 1933, Legg replaced Paul Rotha as head of Strand Films in 1937, where he moved from director to producer.
In 1939, Legg moved to Canada with his former mentor, Grierson, where he launched the National Film Board of Canada's Canada Carries On and The World in Actiondocumentary film series, for which he made numerous films, as both a director and producer. Legg's films include Churchill's Island, which won the first Academy Award for Documentary Short Subject, and Warclouds in the Pacific, which was nominated for the same award.
International work
Postwar, after first working in New York, Legg returned to Britain and worked as a producer for the Crown Film Unit between 1948 and 1950. In 1957, he became chairman of the Film Centre International. Legg later produced documentaries for Shell. Legg also worked with the Anglo-Scottish Pictures Ltd. and the Australian National Film Board, where he was involved in the early-1960s.
Writing
Legg's interest in history was evident. He recounted: "I'd gone into – documentary, but I don't think I was ever very good at it really –I was interested in history. I was interested in international affairs." His interest in history led him to write The Heartland ; dedicated to Grierson. Reviews noted the book, "... gives the grand sweep of European and Asian history in terms of the continual conflict between the great coastal civilizations and the barbarian horsemen from the central Asian steppes."
Family life
Legg married Margaret Amos, daughter of Sir Percy Maurice Amos KBE KC. However, they lived apart for many years.