In 1965 he moved to Buryatia to work at the Dzhidinsky tungsten-molybdenum plant, where over a 28-year period he rose from the position of mining foreman to the head of the enterprise in 1980. Semyonov was elected deputy of the Supreme Soviet of the Soviet Union, sitting in the 11th convocation between 1984 and 1988 as representative of Okinsky, Tunkinsky and Zakamensky Districts, and in 1988 he was nominated deputy chairman of the Council of Ministers of Buryatia. After the dissolution of the Soviet Union he was elected three times as a deputy of the People's Khural of the Republic of Buryatia for Zakamensky District, the first on 30 June 1994, and on 21 July 1994 he was elected to the first of two terms as its chairman, during its first and second convocations. These were particularly significant terms, as it was during the first and second convocations of the Khural that many of the basic laws and state structures of the Republic were codified and enacted. He was elected as deputy for the second time on 5 July 1998, and began his second term as chairman on 20 July that year. Semyonov oversaw several initiatives to strengthen Buryatia's economic standing, such as the building of , and the fourth power unit of the . Other industrial and economic developments included the construction of the Tugnuysky coal mine, the LEP-500 main power transmission line between Irkutsk and Gusinoozyorsk, 150 km of paved road between Mondy and Orlik, and a new telephone exchange in Ulan-Ude. He also participated in the development of a state-owned gold mining enterprise, which later became . On 10 March 1999 Semyonov was elected Chairman of the Inter-Parliamentary Council of Legislative Bodies of State Power of the Agin-Buryat Autonomous Okrug, the Republic of Buryatia and the Ust-Orda Buryat Autonomous Okrug. From January 1996 to December 2001 Semyonov sat on the Federation Council as representative of Buryatia. He sat on several of the Council's committees during his tenure, firstly on the Committee on Budget, Financial, Currency and Credit Regulation, Money Issues, Tax Policy and Customs Regulation from January to February 1996, and from February 1996 as the Deputy Chairman of Committee on Social Policy. In 2002 he began his third term as a deputy of the People's Khural, serving during its third convocation until 2007.