Soldier. Born in Aberdeen, Duff fought with distinction against the French during the Peninsular War (1808-14), rising to the rank of Major General. He was wounded at the Battle of Talavera (1809). He was honoured by Spain and was appointed to the Order of the Thistle in Scotland.
Having succeeded to the Earldom of Fife on the death of his father (1811), he went on to found Dufftown (1817). He served as Member of Parliament for Banffshire (1818-27) and was also Rector of Marischal College, Aberdeen (1820-24).