After a successful career in the Royal Marines, which included a two-year secondment to the United States Marines Corps, three years as an Equerry to Prince Philip and Command of the Royal Marines Officer Training, established himself as a businessman in the 1980s. He was a founding partner in the shipping container company Textainer, a Director of the Debenhams UK HomeCentre firm and a Director of a subsidiary of the Merchant Bank Mercantile House before establishing Christie-Miller Ltd - a training and development consultancy, with the headline Creating the Future. In 2020 he established Working From Home Pro Ltd, specialising in assisting firms and individuals to maximise their productivity and competence in remote working. He is the author of and Duncan also completed two successive Marathons in the Sahara Desert in 2019 in aid of Parkinson's UK.