After achieving a brilliant academic record at the University of Sydney, Evatt served in the New South Wales legislature from 1925 to 1930. He spent 10 years (1930–40) as justice on the high court of Australia and then reentered politics as a federal representative. Appointed attorney general and minister for external affairs when the Labor Party returned to power in 1941, he sought a larger voice for Australia in Allied military decisions in the Pacific. Convinced that the United Nations was essential to Australia's security, he helped write the UN charter, led Australia's delegation to the assembly (1946–48), and served as president of the General Assembly (1948–49). In his association with the UN, he was an ardent spokesman for the rights of small nations.

Evatt assumed Labor Party leadership in 1951 when Joseph Chifley died. He successfully countered Prime Minister Robert Gordon Menzies' attempt to outlaw the Communist Party of Australia, and his opposition to the administration in 1954 over an alleged Soviet espionage case caused the anti-Communist wing of the Labor Party to secede, signalling the decline of the party from national power. Evatt retired from politics in 1960 to become chief justice of New South Wales (1960–62).