The United Artists agency said Kavanagh died Monday from a brain tumor.
Kavanagh's stable of writers included her husband, Julian Barnes, Robert Harris, Margaret Drabble, Ruth Rendell, Joanna Trollope, and Poet Laureate Andrew Motion.
Kavanagh was a leader of last year's walkout by agents at the venerable Peters, Fraser and Dunlop literary agency in a dispute with its owners, CSS Stellar PLC. The agents banded together to form United Artists.
"She was fantastically efficient and just the person you wanted to have represent you," said Harris. "There was no one quite like her really. And she was exotic, like a bird of paradise."
Novelist Blake Morrison said Kavanagh was the finest agent of her generation. "She was completely trustworthy and when you sent her a typescript she would always let you know what she thought in no uncertain terms. She had complete integrity," Morrison said.
"Pat Kavanagh had the glamour to reduce most men and not a few women to slavery," Clive James wrote in The Guardian.
"She was beautiful, clever and loved to laugh, but she could also have a blunt way with a fool. Since most writers are fools, especially about money, a new client was likely to find his dreams being set straight quite early in the relationship."
Kavanagh was born in Durban, South Africa, and met one of her future clients, Prue Leith, at the University of Cape Town, according to an obituary in The Times.
Kavanagh and Barnes married in 1979.
Funeral arrangements were not immediately announced.