John Joseph Cahill was born on the 21st of January 1881 at Redfern, in Sydney, to Irish immigrants Thomas Cahill and Ellen Cahill née Glynn. He was apprenticed as a fitter in 1907, and joined the Amalgamated Society of Engineers, in which he was a branch officer, frequent conference delegate and activist.
In 1917 when the Eveleigh workshops played the central part in the Great Strike, Cahill was dismissed and not re-employed there until 1922. He married Esmey Mary Kelly in 1925.
In May 1925, Cahill was elected to the NSW Legislative Assembly for the seat of St George (later to be renamed Arncliffe), which he would hold until the landslide defeat in 1932. Though he suffered defamation from the Labor Daily, the newspaper of which Lang was the controlling director, he was re-preselected for his seat, eventually won the respect of the Premier and was elected Caucus Whip.
In 1932 Premier Lang was dismissed by NSW Governor Game, and the Labor Party was defeated in a subsequent landslide. Cahill lost his own seat, which he was not to regain until 1935. He was made secretary for public works and minister for local government in the McKell Government elected in 1941; portfolios he used to great effect to improve the State both during the period of total war effort and in the post-war reconstruction phase. Cahill's duties involved responsibility for energy, and in the post-war period the creation of new power projects, including the Snowy River hydroelectric scheme, which all but eliminated the common late 1940s blackouts.
In 1952 Cahill succeeded James McGirr as Premier of NSW, and established a characteristic style as a competent, tough, effective leader with a productive relationship with the Public Service, dominance in Parliament and the enjoyment of confidence both of the trade union movement and business. While the other branches of the ALP suffered greatly from the 1954-1955 split, Cahill was able to use both his personal gifts as a shrewd conciliator and negotiator, and his public status as a Labor Catholic with the ear of Cardinal Gilroy, to ensure that disaffected members were not able punish the Party's fortunes to the same extent as they did in Victoria and in other States.
Cahill set the record for the longest serving Labor Premier, his term lasting from 1952 until 1959, which was only broken by Neville Wran, and has since been broken again by Bob Carr. Joe Cahill died of a heart attack on the 22nd October 1959, and is buried in Rookwood Cemetery.