He managed the Mount Bischoff tin mine before entering politics and served a term as president of the Institution of Engineers, Australia.

He received the highest vote in the state, and was re-elected at the 1925 and 1931 elections, joining the United Australia Party (UAP) upon its formation.

The government's National Insurance Bill 1928 adopted many of the committee's recommendations, but ultimately failed to pass before its defeat in 1929.

[2] Millen "had a particular interest in technological subjects, including the development of the Commonwealth Council for Scientific and Industrial Research".

[2] Millen was a foundation member of the Institution of Engineers, Australia (IEA), serving on its council and as its fifth president in 1924.