Denis Muller was born in New Zealand in 1948 and emigrated to Australia in 1969. He was educated at Rosmini College, Auckland, and at the University of Melbourne.
After three years on suburban newspapers in Auckland, he joined The Sydney Morning Herald as a sub-editor in 1969. In 1978 he joined The Times, London, also as a sub-editor, before returning to take up the position of Chief Sub-editor of the Herald in 1980.
He subsequently held the positions of Night Editor, News Editor and Assistant Editor (Investigations) at that newspaper, until joining The Age, Melbourne, as Associate Editor in 1986.
At both newspapers, his responsibilities including representing the papers as an advocate before the Australian Press Council.
From 1984 until he left newspapers in 1993, he worked closely with Irving Saulwick, one of Australia's leading public opinion pollsters, in the management and writing of the Saulwick Poll which was published in The Age as AgePoll and in the Herald as HeraldSurvey.
In 1990 he was accepted as a mature-age student into the Public Policy program at the University of Melbourne. He completed a Postgraduate Diploma in 1992 and a Master's degree in 1994.
In 1993 he left The Age to take up a position as Group Manager, Communications, at the Board of Studies, Victoria.
In 1995 he established the research consultancy Denis Muller & Associates, and was appointed a Senior Research Fellow in the Centre for Public Policy at the University of Melbourne.
In 2006 he completed a doctoral thesis on media ethics and accountability, and was appointed a Visiting Fellow at the Centre for Public Policy, where he has taught in the Public Policy program since 1997.
He has also taught research methodology at RMIT University, and teaches defamation law to practising journalists through the Communication Law Centre.