John William Roberts was born on 3 September 1885 at Holbeck, Yorkshire, England, to Annie Elizabeth Townsend and her husband, Charles Roberts, a dyer and stuff finisher. Influenced by his father, who was a foundation member of the Independent Labour Party and later served as a union secretary, young Jack joined the first socialist Sunday school in England in 1893, and at the age of 12 began to participate actively in the socialist movement. Later, while working as a presser in the clothing trade, he became a member of the British Amalgamated Association of Clothiers' Operatives.
In 1908 Jack Roberts emigrated to New Zealand, arriving in Wellington on Boxing Day. He settled in Christchurch a few months later and took up work as a presser. Soon after, with his future wife, Agnes Farrar, he joined the Christchurch branch of the New Zealand Socialist Party. Agnes and Jack, who had emigrated together, were married in Christchurch on 23 December 1911; they were to have no children.
The Christchurch Socialist Party affiliated with the nascent Social Democratic Party (SDP) around 1914, and in August that year Roberts became secretary of its Woolston branch. He was vice president in 1915, secretary again in 1916, and after the SDP joined the New Zealand Labour Party he served as president of the branch until 1938. In April 1917, when Christchurch's labour movement vigorously contested the local body elections on an anti-conscription platform, Roberts was elected to the Woolston Borough Council. His foray into politics ended abruptly, however, when he was called up for military service in December that year. A humanitarian, he refused to serve and was sentenced to 18 months in gaol. After the war Roberts, along with other conscientious objectors, was deprived of his civil rights for 10 years.
Unable to participate fully in the political arena, Roberts again involved himself in the industrial labour movement. He had joined the Christchurch Tailoresses' and Pressers' Union on his arrival in Christchurch, and served as vice president (1912–13) and president (1914–18, 1919 and 1921). When the union's secretary, Fred Cooke, died in 1930, Roberts succeeded him. He also became secretary of the Christchurch Tailoring Trade Union and the Christchurch Dress and Mantle Makers' Union.
Roberts believed in the 'One Big Union' concept and advocated the creation of a combined clothing workers' union. When the Christchurch Tailoring, Dressmaking and Other Clothing Trade Employees' Union was formed in May 1935, he became its inaugural secretary. Roberts's power increased as the union extended its coverage over the next six years to become, in 1941, the Canterbury, Westland, Nelson and Marlborough Clothing Trades Union.
As secretary of the New Zealand Federated Clothing Trade Employees' Association from 1937 to 1959, Roberts was able to promote his ideas on the national stage. In 1945 he argued for the amalgamation of local clothing trade unions in Auckland and Dunedin. Local officials were concerned that women members would lose influence in the union, but Roberts considered this secondary to the principle of one trade, one union, one award. He prevailed in Dunedin, but in Auckland a separate women's clothing workers' union was retained. Roberts advocated equal pay for women and in 1946, as propaganda for the forthcoming general election, he wrote a booklet outlining the history of women's struggle within the clothing industry.
Roberts's influence extended beyond the clothing trade. He was president of the Canterbury Trades Council of the New Zealand Federation of Labour (FOL) from 1937 until 1950, and of the North Canterbury Labour Representation Committee (LRC) from 1936 to 1939 and from 1940 to 1949. A well-respected and capable chairman, he was renowned for keeping discussion focused on the subject at hand. In the Court of Arbitration, where he acted as workers' advocate for almost 30 years, Roberts was respected by all sides for his truthfulness, fairness and knowledge of industrial law. He also served on the Industrial Emergency Council (1944) and the Economic Stabilisation Commission (1945–48) as a workers' representative.
A man of integrity, Jack Roberts 'never deviated from his principles'. While other First World War pacifists, such as Peter Fraser, later compromised their anti-war ideals, Roberts did not. He opposed conscription during the Second World War and in 1948–49 actively campaigned against the introduction of peacetime conscription, using his position as president of the Canterbury Trades Council to organise opposition to the Labour government's national referendum. When voters supported the proposal, Roberts resigned the presidency of the LRC and his position on the Labour Party's national executive, which he had held since 1937. Still committed to socialism, he considered that the party had abandoned many of its original objectives. He remained a member, however, convinced that Labour was the only political organisation in New Zealand capable of promoting workers' interests and world peace.
By the late 1940s Roberts had also become somewhat disillusioned with the leadership of the industrial labour movement. In 1950 he resigned the presidency of the Canterbury Trades Council after siding with the watersiders in their walkout from that year's FOL annual conference. The council had also supported the carpenters the previous year, again in opposition to the FOL's national executive.
A staunch opponent of fascism, Roberts was a foundation member of the Society for Closer Relations with Russia, being president of its Christchurch branch for many years. He visited Russia in 1939, after representing New Zealand workers at the International Labour Organisation's conference in Geneva. In retirement he was a member of the Fire Service Council until poor health forced his resignation in 1961. After a long illness Jack Roberts died at Coronation Hospital, Christchurch, on 12 June 1962. He was survived by his wife, Agnes, who had supported and shared his socialist and pacifist beliefs, and had encouraged him to stand by his principles when expediency was advocated by others.