Osborn's wife, Tania, said the lawyer suffered a heart attack Friday and died early Sunday morning, the Charlotte (N.C.) News & Observer reported Monday.
The attorney took on the case of Reade Seligmann, one of three Duke lacrosse players accused of sexually assaulting an escort service dancer, in 2006.
"Kirk stood up for Reade at great personal cost," the Seligmann family said in a statement. "He passionately believed that the truth would emerge."
Tania Osborn said her husband was particularly proud of the fact that despite defending a dozen or more suspects in capital crimes, none of his clients were ever sent to death row.
"He hated injustice. That was the essence of his life," said Ernest "Buddy" Conner, a Greenville lawyer and friend of Osborn who worked multiple cases with him. "He carried a tremendous amount of credibility, but he did it without getting all angry and aggressive and arrogant."
Osborn is survived by his wife and two daughters.