Delahunty was a news journalist for the ABC and Network Ten from 1975 to 1996. She appeared in news and current affairs programs such as Four Corners and The 7.30 Report. She received a Gold Walkley award for the story Aiding and Abetting which was shown and produced by Four Corners in 1983. Aiding and Abetting was about the improper use of Australian aid money in the Philippines.
In the late 1980s, Delahunty—then the chief newsreader for the ABC in Victoria—was parodied by comedian Jean Kittson on the popular TV comedy series The Big Gig, where Kittson portrayed a snobbish, acid-tongued announcer called Veronica Glenhuntly (whose surname was taken from that of the elite Melbourne suburb).
Delahunty was elected to the seat of Northcote in the Victorian Legislative Assembly at a 1998 by-election. Her maiden speech was about the implications of the Fitzgerald report for Victoria, especially in regard to police corruption.
Delahunty was Minister for Education from 1999 to 2002, during the term of the first Bracks Government. Delahunty was the Minister for the Arts from 1999 to 2006, Minister for Women's Affairs from 2002 to 2006, and Minister for Planning from 2002 to 2005. As Minister for Planning, she was responsible for the media presentation of Melbourne 2030, which is known for protecting the green wedges of Melbourne.
Delahunty did not contest her seat at the 2006 state election.
Mary Delahunty is the sister of Victorian National Party MP Hugh Delahunty. Hugh is also a former Victorian Football League player, as is another brother Michael. Her husband of 22 years, the journalist Jock Rankin, died in 2002. She has two children, Nicholas and Olivia. She was a guest on Life Matters (ABC Radio National) 26 August 2010, on the topics of grief, parenting, civic participation and public life, and her memoir: Public life, private grief.
- "Delahunty, Mary Elizabeth". re-member. Parliament of Victoria. Retrieved 2008-05-10.
- "Delahunty, Mary — Australian Women Biographical Entry". Australian Women’s Archives Project. Retrieved 2008-05-10.
- "Fitzgerald Report on Corruption". Hansard. Parliament of Victoria. Retrieved 2008-05-10.
- Delahunty, Mary. Public Life, Private Grief: a memoir of political life and loss. Hardie Grant. ISBN 9781740668583.
- Holmesby, Russell & Main, Jim (2007). The Encyclopedia Of AFL Footballers. BAS Publishing. ISBN 9781920910785.
- "On life, death and treachery". The Age. Fairfax. March 19, 2005. Retrieved 2008-05-10.