Carter and John Artis were convicted twice, in 1967 and 1975, for a triple homicide in a Paterson, N.J., bar. Carter moved to Toronto in 1985 after a federal judge overturned the convictions.
He was the subject of a 1999 film "the Hurricane," in which Denzel Washington played the lead role.
"I have this helpless sensation there's nothing I can do, other than make him as comfortable as possible," said Artis, now his caregiver, noting Carter has been reduced to 90 pounds.
Artis learned of Carter's condition from Fred Hogan, the retired investigator credited with obtaining the recantations of two key witnesses in the murder trials, which led to the overturning of the convictions.
Hogan went to Toronto and got Carter admitted to a hospital in 2011, the New York Daily News said Monday, adding Carter was given only six months to live at the time.
"Rubin has defied people in a lot of things and he's defying death and logic in this one," Hogan said. "He's just a very, very strong-willed individual."