In 1961 he started a job as a law clerk at Haigh, Charters and Carthy, a role that had varied work and clients, including the Communist Party. On 13 March 1967 Lange was admitted as a barrister and solicitor of the Supreme Court of New Zealand. On 3 August 1968, he married Naomi Crampton. He gained a Master of Laws in 1970 with first-class honours, specialising in criminal law and medico-legal issues. Lange practised law in Northland and Auckland for some years, often giving legal representation to the most dispossessed members of Auckland society – he assisted the Polynesian Panther Party (and, by extension, the Pacific Island community) to disseminate legal rights information and legal aid during the '70s dawn raids.