Begley was selected as the Sinn Féin candidate for West Tyrone in the 2018 West Tyrone by-election on 26 February. The by-election had been called following the resignation of the previous Sinn Féin MP Barry McElduff on 15 January. She was elected with a majority of 7,956. Begley was the first female MP to represent the constituency since it was created in 1997. She held the seat in the 2019 general election with a majority of 7,478. She does not sit in the House of Commons per the party's longstanding policy of abstentionism.
