A member of
Sinn Féin since 1996, Doherty was a founding member of
Ógra Shinn Féin and served on its national executive between 1998 and 2001. In the
2002 general election, Doherty ran unsuccessfully in the
Donegal South-West constituency. On 11 June 2004, he ran simultaneously at the
local elections for
Donegal County Council and at the
2004 European Parliament elections. He failed to win a seat in the European Parliament, but was elected to Donegal County Council for the
Glenties local electoral area. Doherty's second attempt to win a Dáil seat, at the
2007 general election, also proved unsuccessful; he received 21.2% of the first-preference vote. On 2 November 2010, the High Court ruled that the government had delayed unreasonably in holding the by-election. In response to the ruling, the government announced that the
Donegal South-West by-election would be held on 25 November 2010. Doherty stood as the Sinn Féin candidate and won the by-election by a substantial margin, earning 39.8 percent of the first-preference vote. On taking his seat in the Dáil, Doherty was appointed Sinn Féin's spokesperson on Finance. However, the Dáil was dissolved on 1 February 2011, at which point Doherty had been a TD for just over nine weeks. Shortly before the
2011 general election, several newspapers alleged that Doherty had misled the public by stating on various Sinn Féin and
Oireachtas webpages that he had formerly worked as a "civil engineer", an occupation that presumes a degree-level qualification. Doherty insisted that he had "always been upfront" about the fact that he had not completed his degree, clarified his educational credentials, and acknowledged that he had qualified as a civil engineering technician and not a civil engineer. In that election Doherty topped the poll decisively in Donegal South-West, attaining 14,262 first-preference votes (32.97%) . Doherty represented Sinn Féin in the Oireachtas delegation that met the
Bundestag's Budgetary and European Affairs committees in
Berlin in late January 2012. It was revealed in June 2012, that Doherty put €8,000 worth of unspent travel and accommodation expenses towards hiring part-time party workers, despite these expenses being supposed to be returned to the Oireachtas under rules introduced in 2010. A report found that he had not breached any expense rules, and cleared him of any wrongdoing. At the
2016 general election, after a redrawing of constituency boundaries, Doherty was elected to the new five-seater
Donegal constituency on the 8th count with 10,300 votes. On 29 May 2018, Doherty was appointed Deputy Leader of Sinn Féin. He topped the poll at the
2020 general election with 21,044 first-preference votes (27.17%), and was appointed leader of Sinn Féin's negotiations team. On 16 June 2022, Doherty clashed with
Tánaiste Leo Varadkar in the Dáil. Doherty attacked Varadkar for being "out of touch" and brought up Varadkar's legal issues with the DPP. Varadkar responded by calling it a "cheap shot" and brought up a 1999 in which Doherty was convicted of abusing a Garda, saying "You abused, mistreated a Garda Síochána. For that you were prosecuted. You were found guilty. Yes, you got away without a conviction because of your age at the time, but you were actually prosecuted. You were arrested. That's what happened to you." At the
2024 general election, Doherty was re-elected to the Dáil on the first count. He received the highest number of first preference votes in the country, at 18,898 (24.66%). ==Personal life==