After graduating from university, Thea Welsh worked in research and administration in Canberra, Adelaide and Sydney. During the 1990s she was manager of the Australian Screen Directors Association.
Welsh achieved great success with her first novel, The Story of the Year of 1912 in the Village of Elza Darzins (1990), winning several awards, including the Commonwealth Writers' Prize for the South-East Asia and the South Pacific region. Supported by a grant from the Australia Council, Welsh wrote her second novel, Welcome Back (1996), continuing the examination of the film industry begun with her first novel. Her first novel deals with the idea of translation and the second explores the life of a celebrity whose false claim to a Tasmanian origin damages her credibility.
Welsh also wrote the novelisations of the films Dating the Enemy, Paperback Hero, and Appropriateness.