David Harley graduated with a BA in Modern Languages from the University of Cambridge in 1971. He began working as a European Parliament (EP) official in 1975 and remained there until his retirement in 2010. He held various posts within the Parliament including: Adviser in the Cabinet of the President Lord Plumb from 1987 to 1989. He was then adviser in the Cabinet of the Secretary-General of the EP, Enrico Vinci from 1989 to 1995. From 1995 to 1997, Harley was Head of Secretariat, Bureau and the Conference of Presidents. From there he was appointed Deputy Secretary-General of the Socialist Group in the European Parliament. In 1999 he became Spokesman of the Parliament and Director of Press and Media, a position he held until 2004. During the latter two years, he was also Spokesman of the President Pat Cox. From 2004 to 2007 he was Secretary-General of the Socialist Group in the European Parliament. His final appointment was as Deputy Secretary-General of the European Parliament from 2007 to 2010.

Harley is currently part of the Advisory Council of the Foreign Policy Center and of the Independent Complaints Board of the UK Labour Party. He was appointed to the Order of St Michael and St George with a CMG (Companion) in Queen Elizabeth's birthday honours in 2011 for "Services to international diplomacy and UK-European Parliament relations". Following Brexit, Harley obtained Irish citizenship.

David Harley's published works include: "Matters of Record - Inside European Politics" (2021), "The Forgotten Tribe - British MEPs" 1979-2020 (2022 co-editor), "Sidetracked" (2019) fiction, plus numerous articles on EU politics.