His first training licence was as private trainer to Major Lionel Holliday in 1958, at La Grange Stables in Newmarket, before moving to West Ilsley at the end of the 1962 season to take over from R. J.
He produced three Epsom Derby winners in Troy (1979), Henbit (1980) and Nashwan (1989), who also won the 2,000 Guineas and the King George VI and Queen Elizabeth Stakes.
Other major winners include Sun Princess, Dayjur, Hethersett, Bireme, Bustino, Longboat, Little Wolf, Petoski, Highclere, Provoke, Prince of Dance, Minster Son, Unfuwain, Dunfermline and Cut Above.
In 1988, he was controversially sacked from his position as trainer for Queen Elizabeth II at West Ilsley by her racing manager 7th Earl of Carnarvon – Hern was recovering from heart surgery at the time.
Later a compromise was reached whereby Hern shared the stable with the new incumbent – William Hastings-Bass (later Earl of Huntingdon) for a year before moving to Hamdan Al Maktoum's Kingwood House Stables in Lambourn.