Alagiah's parents, Donald Alagiah, an engineer, and Therese, were Tamils. In 1961 his parents moved to Ghana in Africa, where he went to primary school. He spent his secondary education at St John's College, an independent Catholic boarding school in Southsea, England, after which he read politics at Van Mildert College, Durham University. Whilst at Durham, he wrote for and became editor of the student newspaper Palatinate and was a sabbatical officer of Durham Students' Union. He worked on South Magazine from 1982 until joining the BBC, where he was the Developing World correspondent based in London and then Southern Africa correspondent in Johannesburg.

In 2004, he returned to his grandfather's original home in Sri Lanka in the aftermath of the Asian tsunami to survey the damage. The family's former home had been destroyed, though an old well George recalled playing at with his sisters as a child was still recognisable, though unsalvageable.

He is married to Frances Robathan.

Alagiah joined the BBC in 1989 after seven years in print journalism with South Magazine. Before going behind the studio desk, George was one of the BBC's leading foreign correspondents, reporting on events ranging from the genocide in Rwanda, the plight of the marsh Arabs in southern Iraq and civil wars in Afghanistan, Liberia, Sierra Leone and Somalia.

He was the presenter of BBC Four News from its launch in 2002; the programme was later relaunched as The World. and in January 2003 he joined the BBC News at Six, which he co-presented with Sophie Raworth until October 2005, and with Natasha Kaplinsky until October 2007. Since 3 December 2007, he has been the sole presenter of the Six O'Clock News. Prior to that, he had been the anchor of the BBC News at One from 1999. Since 3 July 2006, he has also presented World News Today on BBC World News. He is also a relief presenter on BBC News at Ten presenting mainly Monday - Thursday when Fiona Bruce is unavailable.

A specialist on Africa and the developing world, Alagiah has interviewed, among others, Nelson Mandela, Archbishop Desmond Tutu, former Secretary-General of the United Nations Kofi Annan and President Robert Mugabe of Zimbabwe. His other documentaries and features include reports on why affirmative action in America is a 'Lost Cause', for the Assignment programme, Saddam Hussein's genocidal campaign against the Kurds of northern Iraq for the BBC's Newsnight programme and a report on the last reunion of the veterans of Dunkirk.

In 2000, Alagiah was part of the BBC team which collected a BAFTA award for its coverage of the Kosovo conflict. He has won numerous awards including Best International Report at the Royal Television Society in 1993 and Amnesty International's Best TV Journalist award in 1994. He was appointed Officer of the Order of the British Empire (OBE) in the 2008 New Year Honours.

His appearances at literary festivals include Cheltenham, Keswick, Hay-on-Wye and London, and he has spoken at the Royal Geographical Society, the Royal Society of Arts and at the Royal Overseas League. He is on the Board of the Royal Shakespeare Company.

From 2002-2009 George Alagiah was a patron of the Fairtrade Foundation from which in July 2009, he was obliged to resign by BBC Management who claimed professional conflict of interest. Complaints were received at the BBC from the public who were unhappy that Alagiah had been asked to step down. The BBC responded that in keeping with its principles of impartiality, it would be inappropriate for one of its leading journalists to be seen supporting a movement that clearly represents a controversial view of global trade. He has also been actively involved in supporting microfinance as a tool for development, including recent appearances in support of Opportunity International.