Born the son of Irish migrants in Redfern, Cahill worked for the New South Wales Government Railways from the age of 16 before joining the Australian Labor Party.

Being a prominent unionist organiser, including being dismissed for his role in the 1917 general strike, Cahill was elected to the Parliament of New South Wales for St George in 1925.

His term as premier is primarily remembered for his government's role in post-war infrastructure development, which included the commissioning of the Sydney Opera House and construction of the expressway which now bears his name.

[4] Cahill also made his first attempt to enter politics as a member of the Parliament of New South Wales, when he stood as the Labor candidate for the Legislative Assembly seat of Dulwich Hill at the March 1917 election.

[8] On 17 February 1918 his younger brother who also worked in the New South Wales railways, Sapper Thomas James Cahill, of the 4th Field Company (4th Division), Australian Engineers, was killed in action in France.

[11] After a period of difficult unemployment, Cahill found some work selling insurance amongst other temporary jobs, but was re-employed by the New South Wales Government Railways in mid-1922 at the Randwick Tramway Workshops.

[24][12] However, Cahill lost his bid to be re-elected for Arncliffe to United Australia Party's Horace Harper, at the 1932 election, with the electoral tide sweeping out the dismissed Lang Labor government.

Retaining his interest in political matters, Cahill (despite his earlier support of Loughlin) stayed loyal to Lang when many in the ALP had deserted the flamboyant ex-Premier.

[32] A frequent speaker in the assembly, Cahill often addressed his fellow parliamentarians on railway-related matters and lobbied the government (unsuccessfully) for an extension of the Cooks River Tram Line to Arncliffe.

Commencing office during wartime, Cahill's role as Public Works minister was crucial through the direction of various projects to facilitate continued economic growth and in support of the war effort.

[41][42] By 1943, Cahill was already heavily involved in post-war infrastructure planning, with the McKell Government enabling significant investment in new public works projects such as new ports and roads.

"[44] As the Government had predicted, with the end of the war came a great need for new investment in infrastructure, and in 1946 Cahill introduced new bills for substantial projects such as £1.7 million for a new Mental Hospital at North Ryde and £600,000 for works to dredge and improve the Cooks River and Wolli Creek.

[50][51] When James McGirr announced his resignation as premier on the grounds of ill health on 1 April 1952, Cahill, a natural successor as deputy, put himself forward as a candidate to succeed McKell.

[56] It was in November 1954 that Cahill first began to champion the idea of an opera house in Sydney on the site of the old Fort Macquarie Tram Depot at Bennelong Point.

[57] In August 1957, Cahill responded to criticisms that the opera house was an extravagance and inaccessible to the ordinary public by noting that, "the building when erected will be available for the use of every citizen, that the average working family will be able to afford to go there just as well as people in more favourable economic circumstances, that there will be nothing savouring even remotely of a class conscious barrier and that the Opera House will, in fact, be a monument to democratic nationhood in its fullest sense.

"[58] On 5 February 1959, Cahill signed the contract for the first stage of building works for the Opera House with the chairman of Lendlease and Civil & Civic, Dick Dusseldorp, and the managing director of Brederos, Jan de Vries.

Survived by his wife, Esmey, daughters Gemma and Margaret, and sons Tom, John and Brian, Cahill was granted a state funeral and was laid-in-state at St Mary's Cathedral, Sydney.

[63] His eldest son, Thomas James Cahill, was elected to his father's vacated seat of Cook's River at the subsequent December by-election and served as an MP until his death in 1983.