Hargreaves joins in June on a two-year secondment from Ofcom, where he is a senior partner and board member responsible for international activities.
He will be replaced at Ofcom by an FCO employee on secondment - Martin Donnelly, who is currently director general Europe and globalisation.
Hargreaves succeeds Lucian Hudson, who is moving to another FCO special assignment, focusing on developing relationships between governments, NGOs and businesses around the world. Hudson said: 'I worked with Ian at the BBC in the early 1990s and he's an excellent choice, bringing a lot of experience.'
Key issues for the FCO over the coming year include improving relations with the Middle East, particularly Iran, tackling climate change and helping prevent conflict across the world.
Hargreaves said: 'The big demanding issues include relations with the Muslim world and how all of that plays through into domestic issues in the UK. Also, making global institutions work for Britain is hugely important.'
Before Ofcom, Hargreaves was group director of corporate and regulatory affairs at BAA, a former Demos chairman and a member of the Chancellor's Social Investment Task Force.
In his new role, he will be responsible for comms and public advocacy, as well as organising the FCO's public diplomacy network at home and abroad.
He will also co-ordinate the FCO's wider public advocacy tools, working with the British Council and the BBC World Service.