Lord Guthrie was commissioned into the Welsh Guards in1959. He was promoted to Lieutenant in 1961, Captain in 1965 and in 1966 became a troop commander with 22 Special Air Service Regiment serving in Aden, the Persian Gulf, Malaysia and East Africa. In 1968 he became a squadron commander with 22 Special Air Service Regiment serving in the Persian Gulf and the United Kingdom. He returned to the Welsh Guards in Münster in 1970 and was promoted to Major later the same year.
Promoted to Colonel on 31 December 1979, he undertook a tour of duty in Northern Ireland in Spring 1980 for which he was appointed OBE. He then spent two years as Colonel on the General Staff for Military Operations at the Ministry of Defence. Promoted to Brigadier on 31 December 1981, he became Brigade Commander of 4th Armoured Brigade in 1982.
In 1987, Lord Guthrie became Assistant Chief of the General Staff at the Ministry of Defence. In 1989 he was promoted to Lieutenant-General and appointed General Officer Commanding 1st British Corps, and, having been appointed KCB in the New Year Honours 1990, he relinquished his command on 2 December 1991.
Lord Guthrie became Chief of the General Staff (CGS) on 15 March 1994, being advanced to GCB in the Queen's Birthday Honours 1994. As CGS, he was responsible for providing strategic military advice to the British Government on the deployment of troops for the Bosnian War. He went on to be Chief of the Defence Staff in 1997, in which role he advised the British Government on the conduct of the Kosovo War, before retiring in 2001.
He was created a Life Peer, as Baron Guthrie of Craigiebank, in the City of Dundee, in June 2001, and sits as a crossbencher in the House of Lords.
Guthrie is President of several charities, including Action Medical Research, the Army Benevolent Fund, the Federation of London Youth Clubs and the Progressive Supranuclear Palsy Association. He is a Deputy Lieutenant for Dorset, a Knight of Malta and Patron of the Cardinal Hume Centre.