Joseph Cook entered the New South Wales Legislative Assembly as one of the first Labor members in the election of 1891. Feeling constrained by Labor's strict caucus rules, he left the party for George Reid's Free Trade group and the position of Postmaster-General in Reid's government. Like many senior New South Wales politicians of the time, Cook was elected to the first Commonwealth parliament after Federation in 1901. Here he retained his support for Reid. When Reid retired in 1908, Joseph Cook became leader of the Free Trade Party and, in 1909 with Alfred Deakin, formed the first Liberal government. Cook served as Minister for Defence in this government.

In 1913, Cook replaced Deakin as Leader of the Liberal Party opposition and in June of that year became the Prime Minister following the general election. Following a double dissolution instigated by Cook in 1914, the Liberal Party lost the election and Labor regained control of both houses of parliament.

When 'Billy' Hughes left the Labor Party in 1916, a Nationalist coalition government was formed with Cook as Deputy Prime Minister. He served as Minister for the Navy (1917–20) and Treasurer (1920). Cook represented Australia at the Imperial War Conference (1918) and Versailles Peace Conference (1919) and acted as Prime Minister for five months in 1921. He resigned from parliament in November 1921 to become High Commissioner in London.

Joseph Cook returned to Australia in 1927 at the conclusion of his term as High Commissioner. In 1928–29 he headed the Royal Commission into South Australia as affected by Federation. He died in Sydney in 1947.

Initiated in Lodge Independent No. 8 UGL of NSW in Lithgow on 12 February 1892. In London he affiliated with the Royal Colonial Institute Lodge No. 3556, English Constitution where he served as Deputy Master in 1925. United Grand Lodge of England conferred on him the rank of Past Senior Grand Deacon.