Duff was the only son of Sir Robert George Vivian Duff, 2nd Baronet, of Vaynol, and his wife, Lady Juliet Lowther, only child of the 4th Earl of Lonsdale and his wife, Constance Robinson, Marchioness of Ripon. His maternal grandmother was a sister of the 13th and 14th Earls of Pembroke and Montgomery, and a daughter of the Rt. Honorary Sidney Herbert, 1st Baron Herbert of Lea, the half-Russian younger son of the 10th Earl of Pembroke, and a good friend to Florence Nightingale.
He had one sibling, Victoria Maud Veronica Duff (1904—1967, married John Edward Tennant). He was a godson of Mary of Teck (queen of King George V). Exceedingly handsome and with the courteous manners of a true gentleman, he was famed as a host and raconteur.
He inherited the 1,000 acre (4 km²) Welsh estate of Vaynol (occasionally spelt as "Faenol"), the slate of which was the principal source of the family"s wealth.
Surrounded by the estate"s seven-mile-long stone wall, the Duffs lived in Vaynol New Hall, which had been built in 1800. (The medieval Vaynol Old Hall, also on the estate, was lived in by the farm manager and later the estate manager) On reaching his maturity in 1928, Sir Michael assumed the additional surname of Assheton-Smith, only to renounce it in 1945.
He served as High Sheriff of Anglesey for 1950. He then served as Mayor of Caernarvon, High Sheriff of Caernarvonshire (1932) and Lord Lieutenant of both Caernarvonshire and of Gwynedd.
He also wrote a light novel, The Power Of A Parasol.
The Vaynol estate, in northern Wales, close to the Anglesey estate at Plas Newydd, passed out of Duff family hands, the last main portion including the demesne within the walls being sold off in 1984. This came into the family via Mary Assheton-Smith, niece and heiress of the famous squire Assheton-Smith, the celebrated foxhunter.
He was a practical joker, one of his favourite pranks being to dress up as Queen Mary and pay surprise visits to friends - until he bumped into the Queen herself in a neighbour"s hall.
