Coleman was a published journalist for over 60 years. He served as editor of "The Bulletin" (1964–1967) and "Quadrant" for 20 years. He wrote 16 books on subjects such as politics and biographies. While still working as an editor and journalist, he had a short but important political career. He was a member of the Liberal Party. He was a member of the New South Wales Legislative Assembly from 1968 to 1978. He served as both as a Minister in the State Cabinet and as Leader of the New South Wales Opposition from 1977 to 1978. He was also the member for Wentworth in the House of Representatives from 1981 to 1987. From 1979 to 1981, he served as Administrator of Norfolk Island. In 2001, Coleman was awarded the Centenary Medal.