In 1978 Brown was appointed director of the Tasmanian Wilderness Society. In the late 1970s he emerged as a leader of the campaign to prevent construction of the Franklin Dam, which would have flooded the Franklin River valley as part of a hydroelectricity project. Brown was among the 1500 people arrested while protesting during the campaign. He subsequently spent 19 days in Hobart's Risdon Prison. On the day of his release in 1983, he became a member of Tasmania's parliament for the House of Assembly seat of Denison after the Democrats MP Norm Sanders resigned to successfully stand for the Australian Senate; Brown was elected to replace him on a countback. The Franklin campaign was a success after Federal government intervention protected the Franklin River in 1983.