Margaret was born into a wealthy Sydney family on 24 January 1909, the eldest of five children, and grew up in Wahroonga. She attended The Women's College, University of Sydney, where she was the first female student to have a car, and studied medicine. While at university, Margaret became involved with the Christian Student Movement and identified with Christian pacifism. She graduated with a Bachelor of Science and in 1933 married a doctor, T. A. G. Holmes, instead of becoming one. Margaret and Tag Holmes built a large family home in Military Road, Mosman, a suburb on Sydney's Lower North Shore. Here T. A. G. Holmes had his medical practice and they raised six children. At the beginning of World War II, as many Europeans escaped to Australia, the Holmeses established the "50-50 Club", a weekly social evening where "new Australians" could get to know the locals and better integrate into their new society. These cultural events were attended by many European artists and intellectuals, often with musical performances and national dishes from afar. However, as the threat of war entered the Pacific and blackouts and curfews were imposed in Sydney, the Holmeses were informed by the police that these gatherings should end. In 1959, Margaret made a world trip to attend the congress of the Women's International League for Peace and Freedom in Stockholm; She had come to know of this long-standing women's peace organisation in 1940 through articles in The Peacemaker written by . As there was no Sydney branch of WILPF at that time, she joined as an international member. This was to be the beginning of a long and active involvement with WILPF, including founding the NSW Branch when she returned from the congress. Her trip included travelling to Austria for an IFOR conference, to Russia and to India. It was in the 1960s during the Vietnam War when Margaret's activism became most public. She led demonstrations including the walk-out and was a regular campaigner in downtown Sydney, participating in prayer vigils, candlelight vigils, public meetings and leaflet distribution. During this time she also became active in campaigning for Aboriginal rights and nuclear disarmament. Her biography, Margaret Holmes: The Life and Times of an Australian Peace Campaigner, written by Michelle Cavanagh, was published in 2006. Her life has also been documented in various oral histories and other material, some of which is held by the Australian War Memorial.
Honours
In 2001, Margaret was made a Member of the Order of Australia at the Queen's Birthday Honours, "for service to the community through organisations promoting peace, human rights and conflict resolution, particularly as a member of the Women's International League for Peace and Freedom". She had been a reluctant nominee for the award, only accepting it in the hope that it might help to promote WILPF.
Death
Margaret celebrated her 100th birthday on 24 January 2009 at her home in Sydney, in the company of her six children and nearly all of her nine grandchildren and ten great-grandchildren. She died peacefully on 10 September 2009 in Coffs Harbour, Australia, at the home of her eldest son Bill.
