He received his formal education at Fairburn Primary School, Otara Intermediate School and Otahuhu College, then at the University of Auckland, where he graduated in law in 1965.
He gained a Master of Laws in 1970, then practised law in Northland and Auckland for some years, often giving legal representation to the most dispossessed members of Auckland society. He drew many of his clients from among the underprivileged. He was soon recognized as an outstanding courtroom lawyer and a brilliant orator. He entered parliament in 1977, representing the Labour Party in the Mangere electorate. He became deputy leader of the opposition in 1979 and leader in 1983, assuming the office of prime minister after leading Labour to electoral victory in July 1984. Under Lange, the Labour Party broke with some aspects of its social-democratic tradition. It embraced free-market economics by ending controls on interest rates, cutting subsidies to agriculture and state-run industry, and introducing a value-added (sales) tax in order to trim large government deficits.
In foreign policy, it expanded links with the third world, and severed relations with South Africa; it also alienated traditional allies by adopting anti-nuclear policies, which led Australia and the United States to exclude New Zealand from the ANZUS military alliance. In 1987 Labour was returned to office with an increased majority.
In and out of parliament, Lange was a controversial figure. He underwent surgery for obesity and, while prime minister, tried his hand at saloon-car racing. In 1985, defending New Zealand's anti-nuclear stance, he won a celebrated debate at the Oxford Union against the Rev. Jerry Falwell, an American radio and television evangelist. The stock-market crash of October 1987 exposed the weakness of the New Zealand economy. Lange's attempt to reduce unemployment and restructure the economy without afflicting working people brought him under increasing attack by his free-market colleagues. He unexpectedly resigned from office in August 1989 but remained a member of parliament.
Lange was raised in a Methodist family. In the 1960s he worked alongside Methodist theologian Donald Soper at the West London Methodist Mission in England. He was deeply influenced by Soper's interpretation of Christian socialism. Lange later became a Methodist lay preacher.
Lange led Labour to a landslide victory, greatly helped by vote splitting between National and the New Zealand Party. However, before Lange was sworn in as Prime Minister a foreign exchange crisis arose, which led to a constitutional crisis. The New Zealand Dollar was overvalued.
Lange made his name on the international stage with a long-running campaign against nuclear weapons. His government refused to allow nuclear-armed ships into New Zealand waters, a policy that New Zealand continues to this day.
Lange suffered all his life from obesity and the health problems it caused.
On 3 August 1968, he married Naomi Crampton. In 1989 Lange announced in a brief press statement on 10 November that he was separating from his wife of 21 years.
He had three children, Roy, Emily, and Byron (now in their 30s) with his first wife (Naomi) and one daughter, Edith, with his second wife (Margaret). He married Margaret in Glasgow on 12 January 1992.