Her husband was at her hospital bedside when she died Friday.
Born Virginia Pepper in Godalming, Surrey, she married Ranulph, an army officer, in 1970. They had no children. The two had been childhood sweethearts, meeting when he was 12 and she was nine.
Mrs. Fiennes came up with the idea of a voyage around the world across both poles, the London Mirror reported. And, the couple set off on the 35,000-mile journey from London in 1979, which took three years.
In 1987, she became the first woman to receive the Queen's Polar Medal.
In 1998, she said: "I'm not the sort to throw myself on the floor and burst into tears each time he sets off. I married Ran knowing what he's like."
She bred pedigree cattle and sheep at their farm near Exeter.