Sir Philip Barton has been named as the top civil servant at the new Foreign, Commonwealth and Development Office.
The UK’s High Commissioner to New Delhi will start in the role of permanent secretary on 1 September.
He replaces Sir Simon McDonald, who was asked in June to step down ahead of the merger “at the request” of the PM.
Foreign Secretary, Dominic Raab welcomed Sir Philip to the department, calling him “an outstanding public servant”.
Boris Johnson announced in June that he planned to merge the Department for International Development (DfID) – which is responsible for overseas aid – into the Foreign and Commonwealth Office by September.
He told MPs the “long overdue reform” would ensure “maximum value” for taxpayers.
The PM was understood to want someone new in charge of the department, so asked Sir Simon to step down after five years in the role.
Sir Simon said he “fully” respected the decision and the merger was “the right move for our future overseas effort”.
But he was one of four senior civil servants to announce their exit from Whitehall in six months amid reports of growing tensions with top members of the No 10 team.
Sir Philip has held a number of public service roles, including as the Foreign Office’s as director general, consular and security, and as the acting chair of the Joint Intelligence Committee in the Cabinet Office.
He was also the former High Commissioner to Islamabad, deputy head of mission in Washington, and the director of foreign policy and coordinator for Afghanistan and Pakistan.
Sir Philip said he was “delighted” to have been appointed, adding: “I look forward to bringing together our diplomats and development experts to deliver for the people of the UK and act as a force for good around the world.”
Mr Raab added: “Philip is well placed to bring together the talent and expertise of both departments while helping combine our foreign and development policy in a way that is innovative, ambitious and more integrated than ever before.”