Mary Anne Wilson was born in London, England, on 22 September 1820 and died at Blenheim, New Zealand, on 18 July 1901. Her father's name was James Norris, but he assumed the name Wilson when he married Mary's mother, whose surname was Croft. Mary married James Whitney Griffiths, a chemist, in London on 16 December 1841 and they had two sons and a daughter. In August 1849 she and her two younger children left England on the Pekin. They arrived in Nelson in January 1850. Mary Griffiths was described on the ship's passenger list as a widow but it is almost certain her husband was still alive and that she had left him on account of his cruelty. On 5 December 1851 at Nelson, having ascertained Griffiths's death, she married Stephen Lunn Müller, a widowed doctor with four children, who had also come to New Zealand on the Pekin. In 1857 the family moved from Nelson to Blenheim, where Stephen Müller took up the position of resident magistrate. There they were joined by Mary's eldest son.
Mary Müller had a keen sense of the legal and political disabilities of women, based on personal experience. Her first and greatest concern was that, on marriage, women lost all rights to own and control property. Her second concern was that women were not able to vote. In 1864 she met the English women's rights advocate, Maria Rye, who was visiting New Zealand, and from then on closely observed the course of the women's rights movement in Great Britain and the United States. She began, under the pen name 'Fémmina', to contribute articles on women's rights to the Nelson Examiner. Because of her husband's opposing views she had to work 'like a mole'. The editor of the Nelson Examiner, Charles Elliott, was a friend and relative by marriage of Mary Müller. He saw that the articles were widely distributed and soon her influence extended well beyond Nelson.
In 1869 she wrote the first pamphlet on the woman's vote published in New Zealand, An appeal to the men of New Zealand. In it she argued that women should not be discriminated against in law or politics on grounds of their sex, that they had as just a claim to the vote as men, and that without political rights they could not make their full contribution to the progress of the nation. 'How long', she asked, 'are women to remain a wholly unrepresented body of the people?' She urged men to take the initiative in electoral reform and made a special plea to parliamentarians: 'Women's eyes turn in hope – nay trust – on some leading spirits who will not fail them.' In 1870 Mary Müller received a letter of congratulation from John Stuart Mill, to whom she had sent a copy of her pamphlet. Mill urged her to form a committee to work for the vote, but she was unable to act publicly. However, she subsequently met and influenced politicians, notably Alfred Saunders and William Fox.
Acts to protect the property of married women were passed in 1870 and 1884, and in 1893 women won the vote. Mary Müller witnessed these reforms with pleasure and in 1898 she wrote to leading suffragist Kate Sheppard: 'Old & failing, it is cheering to watch the efforts of the younger and abler women striving bravely to succeed in obtaining rights so long unjustly withheld'. In December 1898, seven and a half years after her husband's death, her identity as 'Fémmina' was finally revealed. She then became known, through the writings of Kate Sheppard, as New Zealand's pioneer suffragist.