Simmons & Simmons tax partner Stephen Coleclough gets the top departmental job as head of corporate tax following the departure of Edward Troup.
Coleclough is also appointed chairman of the Chartered Institute of Taxation's indirect taxes sub-committee.
Coleclough, 33, has been with the firm since 1987 and a partner since 1991. He currently heads the firm's tax team.
Troup joins the Treasury as special adviser to ministers on tax issues.