From an early age, Mansell was a radical protester about the status and treatment of Tasmanian Aboriginals within the community. However he discovered that mere protest was an ineffective measure to achieve his aims of land rights and improved conditions and the radical tactics that he and other Indigenous rights protesters employed in the 1970s were abandoned.

In 1972, he and others set up the Tasmanian Aboriginal Centre of which he was chairman and legal manager and he is the secretary of the Aboriginal Provisional Government.

Mansell undertook a degree in law at the University of Tasmania, graduating in 1983. He began a career as a Lawyer, attempting to defend the rights of Aboriginals, whilst pursuing an agenda of reform. Since then, he has become a qualified barrister and solicitor of the Supreme Court of Tasmania, and the High Court of Australia.

In April, 1987, at a conference sponsored by Colonel Muammar Gaddafi's dictatorship in Libya called "A Conference on Peace and Revolution in the Pacific", Mansell spoke to a large international audience.

To gain international attention for the cause of Tasmanian Aborigines, Mansell established an alternative Aboriginal passport. In 1988 he secured recognition for the passport from Gaddafi's regime which declared it valid for travel to Libya. Mansell said he had Gaddafi's support for the establishment of a separate Aboriginal nation.

In 2001 Mansell stated that "there were more phoney than real Aborigines in Tasmania and more than half the voters in the 1996 ATSIC election were not Aboriginal". Mansell's Tasmanian Aboriginal Centre brought court challenges against the claims of Aboriginality of a number of candidates to the Aboriginal and Torres Strait Islander Commission.

In February 2008 Mansell said on Australian radio that although he was happy that then new Prime Minister Kevin Rudd would offer a formal public apology on behalf of all Australians for the treatment of the "Stolen Generations", he referred to it as a "half-measure" if it was without compensation. On the first anniversary of the apology, Mansell said that the apology had not improved the situation of aborigines, nor had the government stopped welfare policies based on race.

Some of the subjects that Mansell has written about include the Australian Constitution, Aboriginal customary law, cultural and intellectual property, the Human Genome Project, land rights and Aboriginal sovereignty. Mansell has written a book Treaty and Statehood: Aboriginal self-determination, published by The Federation Press in November, 2016