Lynch was given just over six weeks to retain the marginal seat of Halifax for Labour having been selected as the party's candidate only at the end of March 2015. Labour's search for a new candidate was triggered by the decision of sitting MP Linda Riordan in February to stand down on health grounds. Her decision, and Lynch's subsequent selection, set up one of the most closely fought contests in Yorkshire in the election. Riordan had held Halifax for Labour in 2010 by a majority of just 1,472 votes, and the seat was considered a key target for the Conservative Party. Lynch was successful on election night and held the Halifax seat for Labour with a majority of only 428 votes over the Conservative candidate. Lynch made her maiden speech in the House of Commons on 9 June 2015. She stated her priorities as being human rights, the UK's relations with Europe and the protection of services at Calderdale Royal Hospital. In the 2017 general election, Lynch increased her majority to 5,376 over the Conservative candidate, Chris Pearson. Lynch was a member of the Environmental Audit Committee from July to October 2015 and was appointed to the Procedure Committee in February 2016. Lynch was appointed as an Opposition Whip in the Commons on 18 September 2015. Lynch is the chair of the All-Party Parliamentary Group on Fairtrade and a Co-Chair of the British Museum group and of the Population, Development and Reproductive Health group. Lynch has also been a member of All-Party Parliamentary Groups on Migration, Britain-Palestine, Labour Friends of Palestine and the Middle East, Art and Health, Kashmir, Policing and Rugby Union. She supported Owen Smith in the failed attempt to replace Jeremy Corbyn in the 2016 Labour Party leadership election. Following the October Shadow Cabinet reshuffle and the firing of Dame Rosie Winterton as Chief Whip, Lynch resigned as a whip. She was reappointed to the front bench by Labour leader Jeremy Corbyn on 3 July 2017, taking on the role of Shadow Flooding & Coastal Communities Minister within the Environment, Food and Rural Affairs team. In the 2019 general election, Lynch was re-elected to the Halifax constituency with 21,496 votes out of 46,458 cast, and a majority of 2,569. She endorsed Keir Starmer in the 2020 Labour Party leadership election. Following Starmer's victory, Lynch was appointed as Shadow Minister for Immigration.
