Matthew Joseph Mack was born at Wellington, New Zealand, on 3 March 1867, the son of Barbara Wilkinson and her husband, Thomas Mack, who farmed in the Ohariu Valley. About two years later the family moved to Auckland, where Joe (as he was generally known) attended St Paul's School and Wellesley Street School. He was an able student but left school at the age of 13 to work on his family's farm near Helensville.
In June 1885 Mack entered the government railway service. As a junior porter at the Helensville station he earned four shillings for a 12½-hour day doing 'everything from car-cleaning to shunting'. The following March a group of Auckland railwaymen, led by Christopher Leek, established the Amalgamated Society of Railway Servants of New Zealand (ASRS). Mack joined on 1 June 1886 and soon became active in its affairs; in his spare time he studied accountancy and law. Several years later he was promoted to guard, and transferred to Frankton Junction, Hamilton. There he met Charlotte Hill, whom he married on 30 August 1892 at St Peter's Church, Hamilton. They were to have seven daughters and two sons.
Mack's rise to prominence in the ASRS began in 1904 when he was elected a delegate to its biennial national conference. In 1906 he helped to establish a branch at Frankton Junction, and was its first chairman. At the 1908 conference he took a leading role in reshaping the society's constitution and was elected general secretary, defeating the long-serving W. J. Edwards. Moving to Wellington, Mack proved himself an outstanding administrator. He reorganised the society's finances, invested shrewdly in property, and successfully managed its journal, the New Zealand Railway Review. He was also a determined advocate: in 1910 and 1911 the ASRS petitioned Parliament for improved wages and conditions. At the 1911 general election he unsuccessfully contested the Parnell seat for the first New Zealand Labour Party.
The ASRS was New Zealand's largest trade union – by 1913 it had over 7,000 members and more than 40 branches – but it was a cautious organisation. Government superannuation, a statutory grading and promotion structure and a strict seniority system made railway workers largely immune to the contagion of industrial militancy. Like many moderate unionists, Mack considered the leaders of the New Zealand Federation of Labour to be dangerous extremists. He led an ASRS delegation to the Unity Congress in July 1913, but they walked out over the issue of strikes. The ASRS also refused to join the waterfront strike later that year, and in return for their neutrality Mack extracted further concessions from the government.
Described by the prominent unionist Jack McCullough as straight, clean and honest, Mack was a devout Christian and an ardent prohibitionist – some said fanatical. He also strongly supported military conscription in the First World War. Dismayed by the New Zealand Labour Party's opposition to conscription, Mack contested the Wellington Central by-election in 1918 against Labour's Peter Fraser. Although he described himself as an independent representing 'reasonable Labour', the Protestant Political Association of New Zealand strongly endorsed Mack as 'a Protestant who can sing the National Anthem, loyal and patriotic'; Fraser, the PPA warned, stood for 'Bolshevikism in New Zealand'. Labour supporters denounced Mack as a traitor to his class and disrupted his meetings. He polled a creditable second, but Fraser won with ease.
The ASRS had loyally deferred wage demands during the war, but when the government refused to grant an increase in early 1920, railway workers struck for the first time in 30 years. The stoppage was timed to disrupt the prince of Wales's tour, and the government quickly conceded. Later that year Mack was elected president of the New Zealand Alliance of Labour; despite ideological differences he worked closely with its militant secretary, 'Big Jim' Roberts. It was also said that Mack's farming experience helped him form a good working relationship with Prime Minister W. F. Massey. In 1922, however, Massey's government cut public servants' wages by up to 10 per cent. After exhausting other means of protest ASRS members voted to strike, against Mack's advice, in April 1924. Engine drivers stayed at work, however, and the strike was crushed within a week. The government forced the ASRS to leave the Alliance of Labour, although Mack remained president until July 1925.
Troubled by illness for some years, Mack resigned as general secretary of the ASRS at the end of 1926 and retired to Ōpua in the Bay of Islands. He wrote an account of the early years for the society's 5Oth anniversary in 1936. After Charlotte Mack's death in 1913 he had married Flora Nisbet (née McMillan), in Wellington on 26 May 1922; they did not have any children. Joe Mack died in Henderson, Auckland, on 18 July 1951; he was survived by Flora, and six daughters and a son from his first marriage. A dedicated and capable unionist, Mack's commitment to Protestantism, prohibition and patriotism placed him firmly on the moderate wing of the New Zealand labour movement.