Harriett was elected as a Member of the UK Parliament in May 2010. Between 2018 and 2019 Harriett served as Minister for Africa and Minister of State for International Development, with responsibility for children, global education and youth.

In 2020 she became the founding co-chair of the International Parliamentary Network for Education, which works to grow political leadership for education.

Harriett has also served as Parliamentary Private Secretary to the Minister for Employment, Economic Secretary to the Treasury and Parliamentary Under Secretary of State responsible for Defence Procurement.

She is currently Chair of the British Group of the Inter-Parliamentary Union, as well as Chair of the All-Party Parliamentary Group (APPG) on Global Education, and a member of the NATO Parliamentary Assembly and a member of both the Treasury Committee and the House of Commons Finance Committee.