Isabella Malcolm was born in Hoxton, London, England, on 22 June 1850, one of five children of Scots parents Jemima Crawford Souter and her husband, Andrew Wilson Malcolm, a brewer's clerk. The family moved to Dublin around 1858. The children shared an Irish nurse and were members of the Presbyterian church. Andrew Malcolm died in 1862 and the family later emigrated to New Zealand on the Matoaka, arriving at Lyttelton on 8 February 1869. After settling in Christchurch the family joined the Trinity Congregational Church, and Isabella (now known as Isabel) became an active member of its Ladies' Association.
On 17 September 1879 at Christchurch she married Henry Ernest May, a draper. He was a branch manager for George Beath, the husband of Isabel's sister Marie. During the first seven years of their marriage Isabel had four children. In 1883 the couple built their large, wooden home in Clyde Road, Riccarton, on land adjacent to the property of Isabel's brother Frank and his wife, Alice. In 1887 Isabel's sister, leading suffragist Kate Sheppard, built her bungalow on the neighbouring property. Isabel May took an interest in Friedrich Froebel's methods of early childhood education, and the children and their cousins shared a nurse and governess on the combined 12-acre estate and orchard.
Following the visit in 1885 of Mary Leavitt, an emissary for the Woman's Christian Temperance Union of the United States of America, Isabel May joined the newly formed Christchurch branch of the New Zealand Women's Christian Temperance Union and was elected superintendent of hygiene of both this and the national union. She was concerned with health issues such as the wholefoods diet, the benefits of exercise and fresh air, and cycling and rational dress. May read her first paper on dress reform to the Christchurch branch later that year, advocating the use of loose-fitting woollen garments that allowed freedom of movement for both children and adults. A member of the Rational Dress Society, London, she read feminist authors such as Elizabeth Stanton and Elizabeth Phelps and excerpts from their works were published on the WCTU page of the Prohibitionist, which Kate Sheppard edited. In 1893, in an introduction to the booklet Notes on dress reform and what it implies by K. Walker and J. R. Wilkinson, she stated that dress design should 'allow individual liberty to express itself by each one wearing a dress that is essentially rational in its design and otherwise in accordance with the wearer's taste', and that 'the best way to ensure grace is to cultivate a healthy supple body'. Isabel and Kate and their husbands became proponents of the knickerbocker suit, played tennis and became keen cyclists.
Isabel May worked closely with Kate Sheppard, who used her home in Clyde Road as an office in the campaign for women's suffrage. May often accompanied her on speaking engagements and helped form various societies sympathetic to suffrage work. She was elected to the first committee of the Christian Ethical Society in 1890 and became the first convener of the health department when the Canterbury Women's Institute was established in 1892. She was active in collecting signatures for suffrage petitions and was a friend and frequent visitor with Sheppard to the home of Alfred Saunders, a prominent suffrage advocate in the House of Representatives.
In 1893 May left Christchurch with her family to go to England, where she continued to associate with feminist groups. She was the New Zealand WCTU delegate to the council of the Women's Liberal Federation, held in London on 10 June 1896, and was a speaker at the annual meeting of the Society for the Abolition of the State Regulation of Vice, of which the foremost campaigner, Josephine Butler, was president.
On her return to New Zealand in 1897, Isabel May was elected president of the Canterbury Women's Institute and also attended the second session of the National Council of Women of New Zealand as the WCTU delegate. She campaigned against the Contagious Diseases Act of 1869 and her speeches were reported in the WCTU magazine, the White Ribbon. She moved to Auckland in 1898 and that year was elected Auckland representative to the WCTU's Leeston Convention of Workers and Friends, where she spoke on the development of temperance work among Maori and on the formation of a youth group, whose numbers had increased under her direction. Her reports as superintendent of the WCTU literature department from 1898 to 1900 showed a similar revitalisation of that department's work.
Around 1900 the family lived for a short while in China before travelling to England, where they settled near London. Isabel May continued to participate in the women's movement and became one of the founding members of the Lyceum Club, a women's organisation for those distinguished in literature, arts or science. She died at her home, Deodara, at Kingston upon Thames, on 1 May 1926, survived by her husband, Henry, two daughters and two sons.