Trewin joined the ABS in 1966 as a Statistics cadet. Between 1992 and 1995 he was the Deputy Government Statistician in Statistics New Zealand  and a Deputy Australian Statistician from 1995 to 2000, when he was appointed as the Australian Statistician.
Dennis Trewin was the driving force behind the ABS's pioneering 'Measures of Australia's Progress' (MAP), a new system of integrated national progress measurement, linking economic, social, environmental and governance dimensions of progress, a project which gained wide respect among other national statistical offices and helped bring about the OECD's Global Project, 'Measuring the Progress of Societies'.--
He holds other senior appointments in Australia such as non-judicial member of the Australian Electoral Commission and an Adjunct Professor at Swinburne University. He has held the office of President of the Statistical Society of Australia.
Internationally, in 2005 he completed a term as President of the International Statistical Institute having previously been Vice-President and President of the International Association of Survey Statisticians. He is a past editor of the International Statistical Review. He is Chairman of the Global Executive Board at the World Bank, Chairman of the Asia/Pacific Committee of Statistics, and board member of Institute of Social Research.
Trewin holds honorary life memberships of the International Statistical Institute and the Statistical Society of Australia. He was listed as one of Australia's Smart 100 in a 2003 poll run by the Australian magazine The Bulletin.
- Tim Colebatch (2006-09-02). "Mining boom fails to deliver". The Age. Retrieved 2007-01-31.
- "Statistical News Tasmania: Australian Statistician Dennis Trewin Retires". Newsletter, Australian Bureau of Statistics. November 2006. Retrieved 2007-01-31.
- "Senior Staff Biographies - Dennis Trewin, Australian Statistician". Australian Bureau of Statistics. Retrieved 2007-01-31.
- "Biography for Dennis Trewin". Observatory Pascal. 2004. Retrieved 2007-01-31.
- "Speaker profile: Dennis Trewin". OECD. November 2004. Retrieved 2006-06-25.
- "The Australian & The Scanlon Foundation presenter profile: Dennis Trewin" (PDF). College of Arts & Social Sciences, Australian National University. Retrieved 2007-01-31.
- The "Bulletin Smart 100" (broken link). The Bulletin. Retrieved 2006-06-25.