In May 2019, Stewart was promoted to the cabinet after the dismissal of Gavin Williamson, replacing the new Defence Secretary Penny Mordaunt in the Department for International Development. The position included full membership of the UK Cabinet, and the National Security Council. It also saw Stewart serve as a governor of the World Bank, the African Development Bank, the Caribbean Development Bank and the Inter-African Development Bank. He was also an alternative governor to the European Bank for Reconstruction and Development.