Buchan is the son of the late William Buchan, 3rd Baron Tweedsmuir and grandson of John Buchan 1st Baron Tweedsmuir the Scottish novelist and diplomat. He has several brothers and sisters including the writer Perdita Buchan.
Educated at Eton and Magdalen College, Oxford he began his career as a Financial Times correspondent, writing from the Middle East, Germany and the United States.
In 1986 he married Lady Evelyn Rose Phipps, daughter of Oswald Phipps, 4th Marquess of Normanby. He has three children, Elizabeth, Nicholas and Rose.