McCarthy qualified as a solicitor in 1994 and worked as a lawyer for Abbey National Treasury Services (1994–1996), Merrill Lynch Europe (1996–1999) and the Labour Party (2001).

In 2005, McCarthy was selected as the Labour candidate for Bristol East through an all-women shortlist[6] and retained the seat for her party at the 2005 general election.

She was appointed a member of the Treasury Select Committee, and was involved in its inquiries into financial inclusion, globalisation and the role of the International Monetary Fund, and the administration of tax credits.

From July 2007 to January 2009, she worked as PPS to Douglas Alexander, the Secretary of State for International Development, before being made a Junior Whip in June 2009.

[15] After being re-elected with an increased majority in the 2015 General Election, McCarthy nominated Andy Burnham in that year's Labour leadership campaign.

She argued in a Spring 2015 interview with Viva!life, a magazine for vegans, that meat should be treated like tobacco, with "public campaigns to stop people eating it".

[20] According to McCarthy, in an article for The Huffington Post: "When the leader's office did venture into Defra territory, they didn't talk to the shadow team".

[28] On 29 June 2022 McCarthy returned to the Labour frontbench as Shadow Minister for Climate Change following the resignation of Olivia Blake.

[31][32] In October 2010, McCarthy admitted a charge of electoral fraud, accepting a police caution for revealing on Twitter the number of postal votes cast per party in her constituency at the 2010 election, and apologised for this action.

[11][33][34][35] In May 2012, McCarthy branded a fellow train passenger a "lager drinking oaf" and suggested he should "have been killed before he could breed" in comments made to her followers on Twitter.