Stella May Henderson was born at Kaiapoi, New Zealand, according to family sources on 25 October 1871. She was the seventh child of Alice Connolly and her husband, Daniel Henderson, who is thought to have started a flax mill and run a store at Kaiapoi. The children were brought up as Presbyterians. Some of Stella's early schooling, begun at the age of three, was in Ashburton, but when she was 11 the family moved to Christchurch to ensure that the children received a better education. At 12 she gained a scholarship to Christchurch Girls' High School where she came under the influence of the principal, Helen Connon. In 1888 Stella won a University of New Zealand junior scholarship to Canterbury College. She completed her BA and received a college exhibition for excellence in honours work in political science in 1892. In 1893 she gained her MA with first-class honours in English and Latin. While studying at university she taught cookery at Christchurch Girls' High.
It was 'while working at Jurisprudence and Constitutional History', she said, 'that the idea first occurred to me of taking a law degree.…I did not know then that the profession was not open to women.' In the 1890s Stella Henderson began to study law. She was greatly helped by the support of local solicitor and law lecturer William Izard, of the Christchurch firm Izard and Loughnan. With the consent of his partner, Izard invited Stella to work in the firm while she studied. To enable her to practise when qualified, he encouraged G. W. Russell, MHR for Riccarton, to introduce a private member's bill so that women could be admitted as barristers and solicitors. The Female Law Practitioners Act was passed in September 1896 and Stella Henderson sat the final examination for Bachelor of Laws in November 1897. She passed in March 1898 but does not appear to have formally graduated.
As a young woman Stella Henderson was described as being of 'more than medium height, with brown curling hair and rosy complexion'. She had 'wide open eyes, looking out with a grave yet frank fearlessness'. She was a founding member in 1896 of the National Council of Women of New Zealand and later of the Australian National Council of Women, and was active in promoting equal pay for equal work for men and women and the removal of restrictions placed on the education, employment and freedom of women. An advocate of local government reform, she campaigned for the extension of universal suffrage to local body elections and for all women and men to be qualified to stand for office. Public ownership of community services was another of her objectives.
In 1898 Stella Henderson was offered and accepted the position of Wellington parliamentary correspondent and political leader writer for the Lyttelton Times. To prevent embarrassment the editor, Samuel Saunders, wrote to the president of the press gallery requesting the seat allocated to the Lyttelton Times for her use. The president responded by holding a vote among gallery reporters as to whether they wanted a woman colleague. Henderson said later that 'many of the journalists were furiously angry that a woman should try to invade this "holy of holies",' and refused to grant her a place. Undaunted she bought a permanent ticket to the ladies' gallery where she balanced her notes on her knee. Her leader articles were written in the ladies' tearoom because it was the only accessible place with a table. Henderson's ostracism became an issue and the matter was referred to the Reporting Debates and Printing Committee of the House of Representatives. The dispute was resolved when a partition was erected providing a special cubicle for her use.
After two years of reporting from Wellington, on 8 March 1900 at Christchurch Stella Henderson married Edwin Frank Allan, an Oxford-educated leader writer with the Evening Post. She resigned from the Lyttelton Times because of a perceived conflict of interest: her paper supported the Liberal party, Edwin's the opposition. In 1903 the couple moved to Melbourne when Edwin was appointed to a senior position with the Argus. Stella continued her career as a journalist. In 1908 she was appointed to the staff of the Argus where she contributed a regular women's feature under the nom de plume'Vesta'. Her writing covered a wide range of topics and her column 'Women to women' was one of the first of its kind in Australia.
In spite of a full-time job, a family of four daughters and her domestic tasks (she was an excellent cook and needlewoman), Stella Allan was also active in a number of community organisations. She worked for the establishment of crèches and free kindergartens, and for improvements to health services in Victoria. In 1922 Edwin died. Two years later Stella was sent as substitute Australian delegate to the assembly of the League of Nations in Geneva.
In 1939 she retired to England, returning to Melbourne in 1947 where she lived until her death, probably on 1 March 1962. She was cremated following a Presbyterian service. Stella Allan was a distinguished member of an outstanding family. A committed feminist, she personally pioneered a path for New Zealand women in both law and journalism. She was sister to Elizabeth McCombs, the first New Zealand woman MP; Christina Henderson, a prominent teacher, editor and social reformer; and Alexander Henderson, editor of the Christchurch Star–Sun.