Hart, 61, died Dec. 24 in La Gomera in Spain's Canary Islands, where the Briton had lived since retiring from the music scene, Sally Hart told The Associated Press.
He had returned to La Gomera three weeks ago after spending a year in Britain receiving treatment, she said.
"In the last few days he became very weak, and had chest pains. His body had suffered greatly from the cancer treatment," she said.
Hart was a star of the 1960s folk scene in Britain, first gaining fame in a musical partnership with singer Maddy Prior in 1966. The duo recorded two albums of "Folk Songs of Olde England," with the versatile Hart backing their singing on guitar, mandolin, dulcimer, banjo and violin.
In 1971, Hart and Prior joined with Ashley Hutchings, who had left the Fairport Convention to form a new band. The new project, at Hart's suggestion, was named Steeleye Span after a character in a Lincolnshire folk song, "Horkstow Grange."
Hart left Steeleye Span in 1983, but appeared at a charity concert with the group in 1995. Last year, he appeared with Prior at a BBC concert in London.
Living on La Gomera, Hart developed his interest in photography. He called the island "my inexhaustible subject" and did his own pictures for his English language guide to the island, published in 2004.
"In what now feels like a previous life, I once spent 16 years as a member of the English folk-rock band Steeleye Span," Hart wrote on his Web site.
"As the Everly Brothers so rightly sang we did it for the stories we could tell. But after a few years of traveling too fast around the world I got myself a camera, a Pentax Spotmatic, in order to provide a more substantial, and accurate, archive of memories," he wrote.
Hart is survived by his wife Connie, and a daughter and son from a previous marriage. Funeral arrangements were not announced.