After many years of backbench service, including a term outside of parliament, Cahill was eventually appointed Secretary for Public Works in 1941 and Minister for Local Government in the government of William McKell in 1944, where he led significant reforms of local government in the state, including establishing a Royal commission in 1945, and passing the landmark "Local Government (Areas) Act" of 1948. Promoted to Deputy Premier in 1949, Cahill became Premier of New South Wales from April 1952 to his death in October 1959. 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.