After entering the New South Wales Parliament in 1913, Lang rose to party secretary and state treasurer (1920–22), becoming premier and treasurer in 1925. During his first ministry he developed Australia's first child endowment plan and sponsored a widows' pension bill. He led the Labor parliamentary opposition (1927–30) and was elected premier in 1930 on a platform opposing the deflationary policies of the federal Labor government.

Refusing to pay New South Wales's interest payments on overseas loans in April 1931, Lang modified his position three months later. Lang's actions deepened the split in the national Labor Party, leading to Scullin's defeat in 1931, and encouraged the development of the right-wing New Guard movement in opposition to Lang's policies. In February 1932, he rejected the new federal statute requiring payment of state revenues to the commonwealth and was dismissed by Gov. Philip Game. He served in the New South Wales Parliament in 1943–49 (although expelled from the Labor Party in 1943), and then briefly in the federal Parliament, before settling into a very long and active retirement. In 1972 the party reinstated his membership.