Flint is of Dutch-Indonesia descent on his mother's side.He studied at Sydney Boys High School before studying law, economics and international relations at the Universities of London, Paris and Sydney, leading to a career in the law and in the academy. Flint was a socialist in his youth. He was Dean of Law at the University of Technology, Sydney from 1987 until 1997. He is currently Second Vice-President and National President for Australia of the World Jurists Association. He is also president of the Federation of Australian Branches of the English Speaking Union, and was for long a board member and former editor of the Australian Branch of the International Law Association.

He was awarded World Outstanding Legal Scholar, World Jurists Association, Barcelona, in October 1991. On 12 June 1995 was made a Member of the Order of Australia "In recognition of service to the print media, particularly as Chairman of the Australian Press Council and to international relations".

Flint was head of the Australian Press Council from 1987 until 1997.

In 1998, he was appointed to the Australian Broadcasting Authority. He resigned from the ABA in 2004, after a controversy over a letter which he had sent to broadcaster Alan Jones in the leadup to Flint's heading the ABA's cash for comment enquiry into commercial broadcasting. As chairman of the ABA, Flint was chairman of the inquiry.. The well-known broadcaster John Laws, also involved in the inquiry, stated he had heard Jones say that he had "instructed" the Prime Minister to reappoint Flint in 2001.

Flint insists that his resignation was "not an admission of guilt." and asserts that he had forgotten the letter. Flint claimed that, despite a thorough Freedom of Information investigation, the one letter proliferated into a 'series of fan letters' in the media. The television program Media Watch, whose pursuit of the story was recognised by a Walkley Award for investigative journalism, provided opportunity for Flint to unambiguously deny the existence of more than one letter. Further, Flint asserted that Laws mistakenly thought Flint was behind more recent ABA action against him. On the contrary, Flint claimed in his book, Malice in Media Land, that he haddissented from the decision to proceed against Laws on the grounds that it was both unjustified and unlawful.

Flint is one of Australia's most prominent constitutional monarchists, in opposition to Australian republicanism. His book, The Cane Toad Republic, was used in the 1999 referendum campaign. This was followed in 2003 by Twilight of The Elites, which was critical of what Flint saw as the elites. In support of Australia's current constitutional arrangements, and the role of the Australian Crown in it, Flint has been National Convenor of Australians for Constitutional Monarchy since 1998, and a board member of the Samuel Griffith Society, a society dedicated to the cause of federalism. He is a patron of the International Monarchist League in Australia, formed by certain Australian Life Members of the International Monarchist League in London, and which seeks to advance the cause of Constitutional Monarchy with a hereditary king as the most ideal form of government for the entire world.