Eight years after his first tilt at state politics, Cahill was elected as one of the members of the NSW Legislative Assembly seat of St George at the 1925 state election, which brought the Labor party back into government under Jack Lang. He was never counted among the close allies of Lang, and remained on the backbench throughout Lang's two terms in government. Cahill, along with half of the parliamentary caucus, supported Peter Loughlin, when the latter challenged Lang for the party leadership in September 1926. With the abolition of the St George constituency in 1930, Cahill ran for Arncliffe and was appointed party whip. 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.