Smith first stood for election as a Labour Party candidate for the University Ward on Lancaster City Council in 2007. She came fifth with 98 votes. Smith supported John McDonnell for leader in the 2007 Labour Party leadership election. In the same year, she was a candidate for Labour Party National Executive Committee Youth Representative. Smith worked as an office manager for the Christian Socialist Movement from 2007 to 2009 before working as a research and constituency worker for three Members of Parliament from 2009 to 2012: Jeremy Corbyn, Katy Clark, and Bob Marshall-Andrews. Smith was the Labour Party candidate for Wyre and Preston North constituency in the 2010 general election, but she was unsuccessful and came in a narrow third behind the Liberal Democrat candidate. In 2020 she told Lancs Live, "I had been called because there was no Wyre and Preston candidate for Labour... I wasn't expecting to win but I was happy to make the case for Labour to the constituents." In 2010–2011, Smith was chair of Compass Youth. In 2011, a majority of the Compass Youth committee, including Smith, resigned in protest at Compass' decision to become a cross-party body. Those that resigned set up a new organisation called Next Generation Labour, which Smith chaired for a period. From 2012 to 2015, Smith worked as a campaigns and policy officer for the British Association of Social Workers. In 2013, she was selected as the Labour candidate to contest Lancaster and Fleetwood constituency at the next election.
On 27 June 2016 Smith entered the Shadow Cabinet as Shadow Minister for Voter Engagement and Youth Affairs. This followed on from a series of resignations of shadow ministers who had lost confidence in Corbyn's leadership. On 6 April 2020, Smith was re-appointed to her shadow cabinet role by the newly elected Labour Party Leader, Keir Starmer. In addition to her other duties, Smith acts as Shadow Deputy Leader of the House, in which role she made her debut at the Despatch Box on 20 December 2016.
Expenses
On 24 May 2016, Lancashire Constabulary announced that an investigation had been opened following allegations that Smith broke election spending laws by spending thousands of pounds more than she declared. In June 2016, Lancashire Constabulary were granted a year-long extension to investigate Smith's election expenses, and in November 2016 they cleared Smith of any wrongdoing.
Personal life
Smith married her partner of eleven years, Ben Soffa, in September 2016. Soffa has worked as head of digital organising for the Labour Party since 2015. In July 2018, Smith gave birth to the couple's first child. She identifies as bisexual. In 2010, Smith was diagnosed with Postural Orthostatic Tachycardia Syndrome, which she says has affected her energy levels, and which she uses medication to control. Smith is a Methodist.