Born in Portsmouth, UK in 1966, he grew up in Portsmouth, Marple and Manchester. He attended Harrytown Comprehensive School in Romiley, Cheshire and St Bede's College, Manchester. He entered Britannia Royal Naval College, Dartmouth in January 1985 and passed out the following August. He was awarded a First in Modern History from Corpus Christi College, Oxford in July 1988. Before becoming a journalist, Harnden was an officer in the Royal Navy, retiring in the rank of Lieutenant in 1994. He married in 2006 and currently lives with his wife Cheryl, their daughter Tessa (born June 2007) and son Miles (born January 2009) in McLean, Virginia.

Northern Ireland

Harnden began his career in journalism initially as a theatre reviewer at the Edinburgh Fringe Festival and a writer of obituaries. He joined the Daily Telegraph in 1994 as a home news reporter before being posted to Belfast as the newspaper's Ireland Correspondent in 1996. He subsequently covered the Good Friday Agreement and the Omagh bombing of 1998. The culmination of his work in Northern Ireland was the publication of the Bandit Country: The IRA & South Armagh, which was critically acclaimed and sold more than 100,000 copies.

United States and Middle East

From 1999 to 2003, Harnden was Washington Bureau Chief of The Daily Telegraph. He became Middle East Correspondent of the Daily Telegraph in October 2003 and was based in Jerusalem but travelling extensively throughout the region. Harnden spent much of 2004 and 2005 covering the war in Iraq. He was a "unilateral" reporter during the siege of Najaf in August 2004 and three months later was embedded with the US Army's Task Force 2-2 during the battle of Fallujah.

Harnden joined The Sunday Telegraph in January 2005 as the newspaper’s Chief Foreign Correspondent. He reported from Iraq, Afghanistan, Israel, the Occupied Palestinian Territories, Lebanon, Bahrain, Zimbabwe, Mozambique, Austria, Italy, Estonia, Saudi Arabia, Pakistan, the United States and Thailand. In May 2005, he was imprisoned in Zimbabwe for 14 days after being arrested at a rural polling station on the day of the country's parliamentary elections and deported following acquittal on a charge of "practicing journalism without accreditation". He returned to Washington in May 2006 as a correspondent for The Sunday Telegraph and in October 2006 became U.S. editor for The Daily Telegraph. Since 2009, he has written a weekly column for The Sunday Telegraph entitled "Toby Harnden's American Way".

As well as the Telegraph titles, he has worked for The Leith Leader, The Scotsman, Western Morning News and The Independent. He has had articles published in The Guardian, The Wall Street Journal, The Sun, The Spectator, The Evening Standard, The Literary Review, East End Life, The Sunday Business Post and Conde Nast Traveller. A regular broadcaster, he has appeared on CNN, Fox, CNBC and the BBC as well as numerous other television, radio outlets; he blogs for The Telegraph as well. Harnden is represented by literary agent Julian Alexander of Lucas, Alexander, Whitley in London. He is working on a book about the deployment of the 1st Battalion, Welsh Guards to Helmand, Afghanistan in 2009, due to be published in the UK by Quercus Books on 3 March 2011.