Greig was born in Broughty Ferry, Scotland in 1880, one of eight children of textile merchant and higher education advocate Robert Greig, and his wife Jane. She attended school in Dundee, Scotland, before the family moved to Melbourne, Victoria, Australia in 1889.
From 1894 she attended the Presbyterian Ladies' College, Melbourne, but decided to leave school after 1896 and enrolled in an arts/law degree at the University of Melbourne in 1897, the first woman to enter the Faculty of Law, and indeed the first to enter any law school in Australia. Although the male students at the Law school were initially opposed to her studying there, they voted at the end of Greig's first year that women ought to be allowed to practise law.
Greig completed her Bachelor of Arts degree in 1900 (although she did not formally graduate until 1904), and on 28 March 1903, graduated with her Bachelor of Laws degree, the first woman in Victoria to do so, and only the second in the country, after Ada Evans who graduated the previous year from the University of Sydney. Greig received third-class honours for her degree, placing her second in her year level.
The rules of practice in force at the time did not comprehend female lawyers, and there was no precedent of women becoming lawyers, in any of the Australian states. Evans had been denied admission to the New South Wales Bar after she graduated on the basis of her sex, forcing her to campaign for the rules of practice legislation to be changed to specifically allow women. Greig and her supporters had already conducted a campaign in Victoria, and in April 1903 the Parliament of Victoria passed the Women's Disabilities Removal Act 1903, nicknamed the "Flos Greig Enabling Act", to specifically allow women to practise.
Greig completed her articles of clerkship, and on 1 August 1905 was admitted to the Victorian Bar, becoming the first woman admitted to legal practice in Australia. A short time later, she was the first woman admitted to the Law Institute of Victoria. She commenced work as a self-employed solicitor in Melbourne. One of her first jobs was for the Women's Christian Temperance Union, drafting their proposed amendments to the Children's Court Act 1906 (Vic), the legislation which established the Children's Court of Victoria.
Greig later worked as an employee of Cornwall Stodart, a firm of solicitors in Melbourne. In about 1930, Greig moved to the town of Wangaratta in north-east Victoria, where she worked in a firm of solicitors led by Paul McSwiney. During this time she toured the countryside around Wangaratta promoting adult and tertiary education. Greig retired in 1942, and moved to Rosebud on the Mornington Peninsula. Greig lived in Rosebud until her death in 1958 in Moorabbin, aged 78.
Greig had four sisters and three brothers. While two of her brothers followed her father into the family textile business, the women of the family blazed their own trails. Sisters Jane and Janet were two of the first women to study medicine in Australia, at the University of Melbourne, Jane a pioneer in public health and Janet the first anaesthetist in Victoria. Another sister, Clara, founded a coaching school for university students. The youngest sister, Stella, followed Flos into law, graduating from the University of Melbourne with a Bachelor of Laws degree on 8 April 1911. Stella died of tuberculosis less than two years later, aged 24.
- Haultain, Lynne (1997-08-12). "Women in the Law". The Law Report. Radio National. Retrieved 2006-08-13.
- "Greig, Grata Flos Matilda (1880 - 1958)". Australian Dictionary of Biography. 9. Canberra: Australian National University. 1983. Retrieved 13 August 2006.
- Milburn, Caroline (8 February 2000). "Young Lawyers Mostly Women". The Age.