McCourt (pictured) joins Brunswick at the end of September from the UK Treasury, where he was chief of staff and special adviser to former chancellor Phillip Hammond MP, providing advice on a range of issues, including Brexit, financial services and healthcare.
He also served as a special adviser to Hammond during his time in the Foreign and Commonwealth office, and previously worked in the office of Andrea Leadsom MP.
McCourt said: "At a time of so much uncertainty in global business, I’m excited to be joining the leading strategic advisory firm, and to help clients play a more successful role in society."
His earlier career included spells working in European equity strategy for Credit Suisse and JPMorgan Chase, as well as consulting at The World Bank and KPMG in Washington DC.
Brunswick UK managing partner Simon Sporborg said: "Duncan’s wide range of talents working in financial institutions and advising on economic policy issues, as well as his international perspective, will be a huge benefit to both UK and overseas clients."