Mann was admitted a solicitor in the King's Bench Division and set up practice in Cannon Street, London.

At Captain (Sir) John Hindmarsh's request, Mann was appointed the first South Australian Advocate–General and Crown Solicitor, this included a position on the South Australian Legislative Council.

[1] Mann sailed in the Coromandel, arriving at Holdfast Bay on 12 January 1837.

He resigned his office on 17 November 1837 due to a dispute with Governor Hindmarsh.

[2] Mann was a partner with Edward Castres Gwynne for a time, and became master of the Supreme Court of South Australia in 1844, and acting judge in 1849.