Catherine Campbell Sword was born on 15 August 1881 in Glasgow, Scotland, the daughter of Margaret Christina Neilson and her husband, William Baird Sword, a journeyman iron-fitter. After attending Rockvilla school she worked in the weaving industry. On 23 March 1900, at Glasgow, she married Charles Stewart, a foreman iron-fitter. By the age of 24 she had three sons.
A staunch Presbyterian, at 16 Catherine had begun to question why 'little girls with whom I came into contact…did not have good clothes to wear’. Now she began to work for Labour candidates in Glasgow and attended meetings of the Glasgow City Corporation, becoming known as ‘the silent lady in the gallery’. To extend her knowledge of the labour movement she went to evening lectures at the University of Glasgow and to Sunday tutorials on economics taken by the socialist John Maclean. She was a founder of the Women’s Co-operative Guild, which aimed to educate and assist members to improve themselves economically. A member of the militant feminist Women’s Social and Political Union, she was taken into custody three times during demonstrations, but on each occasion was sent home, possibly because of her fragile appearance. However, she fell out with the union because it advocated a property-based franchise. During the First World War she helped the dependants of enlisted men.
In 1921 the Stewarts came to Wellington, where Charles worked for the government railway service. Catherine became the first secretary of the Wellington After-care Association, established to mind intellectually handicapped children, in 1928 and represented it on the National Council of Women of New Zealand. She also became active in the Wellington women’s branch of the New Zealand Labour Party and was the first president of the party’s Melrose–Houghton Bay branch.
As economic conditions deteriorated in the late 1920s and early 1930s, Catherine Stewart actively promoted the co-operative movement, which gave full control of its stores to members, returning profit partly as dividend and partly as a fund for educational and social work. In April 1934 she helped to establish the Women’s Central Co-operative Committee in Wellington, and when the Wellington South Co-operative Committee was set up a month later, she became its first president. It held lectures, many of them on socialist economics, at fortnightly meetings throughout the year. Stewart also helped found the Wellington District Co-operative Society, which had a grocery store in Cuba Street.
In 1935 New Zealand’s first woman MP, Elizabeth McCombs, died, and the following year Catherine Stewart helped to found a club in her memory. Its goal was to help women to improve their knowledge of and ability to deal with public questions. In 1938 Stewart's friends persuaded her to accept nomination as the Labour Party’s parliamentary candidate for the new seat of Wellington West. At the party’s annual conference in April, her eloquent speech was a highlight and soothed the fears of reactionaries: ‘I am not speaking as a feminist but as a woman who wishes to stand shoulder to shoulder with our men'. She joined a delegation of women to the minister of justice, H. G. R. Mason, asking that courts be cleared and publicity restricted in cases dealing with domestic matters, the custody of children or sex crimes.
At the election in October Stewart won the seat by 956 votes from the long-serving MP R. A. Wright, then standing as an Independent National candidate. Until 1941, when she was joined by Mary Dreaver, she was the sole woman in Parliament and was positioned in full view of the ladies' gallery. Slender, with white hair, Catherine always dressed smartly in the House. She was described as honest and sincere, although her grandson remembered her as strong-willed, stern and rather dour in comparison with her husband, who had a lively sense of humour. Meanwhile, Charles Stewart had retired from the railways to take charge of the household.
Believing that she had a responsibility to be the ‘Member for Everywoman’ and to concentrate on those issues traditionally associated with her sex, Catherine Stewart strove to protect and further the interests of women, children and those in need. She urged that the working conditions and status of domestic workers be improved, and advocated the establishment of a meeting house for Wellington Maori. In 1940 she helped convene a meeting of delegates from national women's organisations to establish the Women's War Service Auxiliary. Because youth employment had increased under war conditions, she asked that no child under 14 be exempted from school and demanded better accommodation for the young women who came to the city to do essential work. A Christian socialist, Stewart strongly supported the 1938 Social Security Act, but asked for the family allowance to be extended to cover all children of poorer families.
Despite repudiating feminism during the election campaign, Catherine Stewart supported the 'first wave' feminist ideals associated with the Women’s Social and Political Union and the NCW. She saw the state as a home enlarged, arguing that 'women are greater realists than men in economic matters. Budgeting for a home is a much more difficult task than men realize'. Continuing the campaign for the economic independence of married women, she stated that there ‘is nothing so degrading…as to be financially dependent upon another person’. She suggested that every married woman be paid a benefit by the state, or that a fixed proportion of her husband's salary be regarded as legally hers. She also wanted more women police and justices of the peace, and supported calls for women to become jurors and magistrates. Attacking the sexual double standard, she asked: 'Why is it that, when a house of ill fame is raided, the girl or woman who is present is punished by the law, while the man who was with her goes scot-free?’
However, her rigid and doctrinaire beliefs and unwillingness to compromise contributed to her downfall. Stewart accused the opposition of being 'closely associated with vested interests’, and financiers and wealthy businessmen of engineering the war. She condemned the New Zealand Branch of the British Medical Association as a 'glorified union' controlled by a few 'mercenary heads’, and attacked the upper ranks of the clergy for backing them. Her call to abolish private property was simply too revolutionary for most New Zealanders, who either owned or aspired to own land. Even within the labour movement her extremist views struck discord, and she did not hesitate to criticise her own government when its actions did not fit with her principles. A supporter of John A. Lee, she was one of six MPs who voted against his expulsion from the Labour Party in 1940.
Public disapproval of Stewart was heightened by the actions of her two younger sons, who were conscientious objectors during the war. She was subjected to 'much vilification’ because they were in a military defaulters’ camp. As part of a general swing against Labour in the 1943 election she lost her seat to the New Zealand National Party’s C. M. Bowden, who gained 7,625 votes to her 6,442. The following year she was an unsuccessful Labour candidate for the Wellington Hospital Board. She was also unsuccessfully nominated for a place on the Legislative Council by the party’s Karori branch. Soon afterwards she moved with her husband to a chicken farm near Geraldine, South Canterbury, close to their eldest son. Charles died in 1948 and two years later Catherine Stewart returned to Glasgow with her two younger sons. She died there on 2 April 1957.