In 2002, he was elected to the Legislative Assembly of Manipur, as the Democratic Revolutionary Peoples Party candidate in the constituency Heingang. He later joined the Indian National Congress. In May 2003 he was appointed Minister of State of Vigilance in the Manipur state government. In 2007 he retained his Assembly seat, contesting on behalf of the Indian National Congress. He was later appointed as the Minister of Irrigation & Flood Control and Youth Affairs & Sports in the State Government. In 2012 he again retained his Assembly seat for the third consecutive term. In September 2015, Biren stated that the recently passed bill in the Manipur Legislative Assembly to protect indigenous peoples would not harm any community in the state. In October 2016, Biren resigned from the Manipur Legislative Assembly and the Manipur Pradesh Congress Committee, this came after revolt against Okram Ibobi Singh, the then Chief Minister of Manipur. Singh formally joined the BJP on 17 October 2016 and later became the Spokesperson and Co-convener of the Election Management Committee of BJP Manipur Pradesh. He won the 2017 Manipur Legislative Assembly Election from Heingang Assembly Constituency. In March 2017, he was elected as leader of the BJP Legislature Party in Manipur and with a majority of MLAs having been presented to the Governor, he was sworn in as Chief Minister of Manipur on 15 March 2017. He is the first ever BJP Chief Minister in Manipur. In January 2018, he lays the foundation stone for the new academic building of Manipur Public School. The project is undertaken by Manipur Minorities and OBC Economic Development Society. The cost of the project is estimated to be Rs 10.80 crore under the Union Ministry of Minority Affairs and will include new classrooms, headmaster’s room, common rooms, library, laboratories, toilets and separate hostels for boys and girls. In April 20, 2018, Singh launched the First State-Level Ginger Festival at Parbung in Pherzawl District. The festival will remain one of Singh's legacy in promotion of agriculture in the hill district which is one of the most backward in the state. Farmers have reported that since 2018 they are able to sell their organic ginger throughout the year. During the 2020 COVID-19 pandemic in India,Manipur became the third State to declare itself coronavirus free after the two patients who had tested positive successfully recovered from the virus On 17th June 2020 9 MLAs supporting N.Biren Singh led government revolted against him and withdrew support from his government blaming him. Currently N.Biren Singh is running Minority Government in state