Lady Harewood's death was announced by her step-son, the Earl of Harewood, David Lascelles.
In a statement, he said: "I am very sad to announce the death of my step-mother Patricia, who died peacefully after a long illness on 4th May 2018. She was at her home, Harewood House, where she had lived during her long marriage to my father and where, since his death in 2011, she has continued to live, in a house that they both loved.
"My thoughts are especially with her sons Michael Shmith and Mark Lascelles, with her brother Barry Tuckwell and with her wider family both here in the UK and in Australia, where she was born and lived the first part of her life.
"Her funeral will be held here at Harewood and full details will be announced in due course."
Lady Harewood was the widow of George Lascelles, the seventh Earl of Harewood.
The Earl, a first cousin to The Queen, died in 2011.
Lady Harewood was a successful violinist and fashion model. A football fan, she was last year appointed honorary president of Leeds United Football Club, a position her late husband had held for 50 years until his death.