She was born Lavinia Strutt, daughter of the third Baron Belper and his wife Eva, later Countess of Rosebery, in London in 1916, and had racing thrust upon her from an early age. Her first influences included her stepfather the sixth Earl of Rosebery, a very successful owner and sometime Steward of the Jockey Club, and the wife of the trainer George Lambton.
She was a keen hunter and point-to-point rider. Her colours were infamous as they were like those of Lord Derby (black, white cap), only with a cartoon of Mickey Mouse on the front and back. When she became engaged to the Duke of Norfolk, Disney wrote to her to say that the company had not given her permission to reproduce the cartoon character.
She married the 16th Duke in 1937. Her husband became an influential figure in racing administration, a Steward of the Jockey Club and vice- chairman of the Turf Board, and served as the Queen's representative at Ascot for 27 years. The Duchess herself was closely involved with the running of his stables. When their private trainer Victor Gilpin joined the Army in the late 1930s, she played a key role in the training of the horses, although women at the time weren't allowed to hold a licence.
Her marriage saw her take a number of prominent roles outside racing; not least when she stood in for the Queen during rehearsals for the Coronation of King George VI. In 1990 she became the Lord Lieutenant of West Sussex. She was the first non-royal woman to hold such a post in Britain. She was also the first Lady Companion of the Order of the Garter in its 640- year history, preceding by five years Baroness Thatcher's appointment to the Order in April.
She did much work for charity. She was Patron of Riding for the Disabled and President of the National Canine League. She once estimated that she had been involved with 150 charities, including Sussex branches of the Association for the Disabled, the Spastics Society, and the Spina Bifida Association.
It was in 1942 that the Norfolks' horses were first trained in the beautiful surroundings of Arundel Castle, from where John Dunlop has trained since 1966. The first trainer appointed there was Gordon Smyth, who received invaluable assistance from the Duchess.
The best horse to run in the Duke of Norfolk's colours was Ragstone, who helped him achieve a long-term ambition by winning the Ascot Gold Cup in 1974. His victory was thanks in part to tactics drawn up by the Duchess. She knew that Ragstone was not certain to stay the two-and-a- half-mile trip and so introduced a pacemaker to make sure that Ragstone's pace was slow, rather than the traditional role of a pacemaker to ensure a strong, steady gallop throughout a race.
As well as Moon Madness, the Duchess's best horses included the 1976 Free Handicap winner Man of Harlech, and Moon Madness's half-brother Sheriff's Star, trained by his owner's daughter Anne (in her own right Lady Herries of Terregles), married to the cricketer Colin Cowdrey. Sheriff's Star's best performances came when winning the King Edward VII stakes at Royal Ascot and the Great Voltigeur Stakes at York.
Lavinia Mary Strutt, racehorse owner and breeder: born London 22 March 1916; President, National Canine Defence League 1969-75; President, Riding for the Disabled 1970-86 (Patron 1986-95); CBE 1971; Lord-Lieutenant of Sussex 1975-90; LG 1990; married 1937 Bernard, 16th Duke of Norfolk (died 1975; four daughters); died Arundel, Sussex 10 December 1995.