In April 2007, McCarthy was appointed Parliamentary Private Secretary (PPS) to Rosie Winterton, Minister for Health Services, and helped her steer the Mental Health Bill through the Commons. 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. She is chair of the South West Group of Labour MPs, secretary of the All-Party Parliamentary Group on Somaliland, a member of Labour's National Policy Forum, and lead contact for the End Child Poverty campaign among Labour MPs in parliament.