Margaret Malcolm was born at Richmond, Nelson, New Zealand, on 17 December 1848. She was the youngest of five children of Margaret Barrie and her husband, Andrew Malcolm, a wheelwright. Her parents were Presbyterians who had recently immigrated from Scotland. Margaret was educated in Nelson at an academy for young ladies.
On 22 November 1864 at Richmond she married James Selig Siegfried Caro (also known as Jacob Selig Caro), a Polish Jew who had trained in Berlin and Melbourne as a physician. For the next 16 years they lived and worked in isolated mining and farming communities in South Canterbury, Westland and Marlborough. Along with her husband, Margaret Caro sewed up wounds, set bones and administered dental treatment. She was reputedly over six feet tall and her physical strength enabled her to extract teeth with speed and skill; she became well-known for this ability. As an accomplished horsewoman she willingly rode long distances to give treatment. Between 1869 and 1876 she had three sons.
In 1881 Margaret Caro was the first woman to be listed on the Dentists' Register of New Zealand. The family settled in Napier where she established and ran her own dental practice until 1909. She was the only woman to attend the first conference of the New Zealand Dental Association held in Dunedin in 1890, at which she supported a proposed system of registration and qualification for dentists. Her own qualifications have not been established.
Margaret Caro became a Seventh-day Adventist in 1888 as a result of the mission conducted in Napier by Arthur Grosvenor Daniells, an American evangelist. Although unorthodox, the teachings of the church were in accord with her own views. Adventists held progressive attitudes to women, in 1881 voting for their ordination to the ministry, and their beliefs about health and healing had a rational basis.
In the following years, Caro began to write and lecture on food reform. She argued that vegetarianism was a natural and healthy practice, and held that the slaughter of animals degraded humanity. This opinion was based on practical experience: she frequently visited slaughterhouses, assessing conditions and inspecting offal to ensure that tubercular carcasses were not being sold to the public. Eating meat, she suggested, could lead to ill health and moral failure: 'Flesh foods do arouse the animal in man, their consumption rendering it more difficult for him to maintain a high degree of self control.' Alcoholism and related social problems were a possible consequence. In all these views she was strongly influenced by Ellen White, an American Adventist who visited New Zealand in 1893 and who became a friend and colleague. Ellen White argued that caring for the body was a spiritual duty closely related to caring for the soul.
An activist as well as a theorist, Margaret Caro joined a church programme to rehabilitate alcoholics by taking them into her own home. In 1898, concerned for the victims of male drunkenness, she helped establish the Bethany Rescue Home for women and children. Believing education to be an effective means of change she used her income as a dentist to sponsor young Maori and Pakeha temperance missionaries at Seventh-day Adventist colleges in America and Australia. Maui Pomare, one of the students she supported, was later influential as a medical practitioner and politician. In addition, she appears to have helped pay for the education of her children. Two of her sons, one a doctor and the other a dentist, followed her example and worked for Adventist causes; in 1911 the family opposed military conscription for members of the church.
Margaret Caro became a member of the progressive women's organisations of her time where her views on diet found a sympathetic audience. In 1902 at the conference of the National Council of Women of New Zealand in Napier she presented a paper on vegetarianism which was later published in the White Ribbon under the title 'Man's natural diet'. She supported the reforms promoted by the New Zealand Women's Christian Temperance Union including the extension of the franchise to women.
After the death of her husband in 1902 she trained in California as a Bible worker and spent time at Avondale College, the Seventh-day Adventist centre in New South Wales. On her return to New Zealand she became active as a Christian worker in gaols. In 1903, in a seemingly paradoxical move, she joined the campaign against the introduction of Bible studies in schools. She was aware that small sects would not have the resources to teach in schools, and believed, with other Seventh-day Adventists, that a secular education was preferable to religious instruction by large denominations, notably the Anglican church. Under her leadership the Napier Seventh-day Adventists posted over 1,000 tracts on the issue to influential people throughout New Zealand.
Margaret Caro lived mainly in Napier until about 1936, when ill health forced her to move to a son's home in Wellington. Her commitment to the church, in which she held a number of offices, lasted until her death in Wellington on 19 May 1938.
Margaret Caro was unusual among New Zealand middle-class women reformers of her time in that during her married life she worked continually and with financial success in a male-dominated occupation. Her achievement was due in part to her personality, which, according to a contemporary, combined 'the cordial manner of the colonial…with the solidity of character so characteristic of the Scotch nation'.