Rubin "Hurricane" Carter, the former U.S. professional boxer turned advocate for the wrongly convicted, has died. He was 76.

Carter, who spent almost 20 years in prison after being wrongfully convicted twice for a 1966 triple homicide, died in Toronto following a battle with prostate cancer. John Artis, Carter's long-time friend and co-accused, said the former boxer died in his sleep Sunday morning.

Artis left the U.S. to act as Carter's caregiver in Toronto after the former boxer was diagnosed with cancer three years ago.

During the last few months of his life, Carter had tried to accomplish as much as he could and had come to terms with his imminent death, Artis said.

"He didn't express very much about his legacy. That'll be established for itself through the results of his work. That's primarily what he was concerned about – his work," he said of his friend who helped to found the Association in Defence of the Wrongly Convicted.

"He was a very selfless person."

Carter and Artis were both convicted in the 1966 murder of three people at a New Jersey bar. Their convictions were overturned in 1975, but they were both found guilty for a second time one year later.

After a lengthy legal battle, Carter was freed in 1985, when a judge overturned the second conviction.

He later moved to Toronto, where he met Canadian Boxing Hall of Famer Charles "Spider" Jones.

The former professional fighter said Carter, who was known for his "explosive" punches in the ring, had a powerful personality.

"When he delivered his motivational speeches…he was very, very blunt. He did a lot of good," Jones told CP24 on Sunday.

Carter's legal troubles, however, had left its mark on the former fighter. Jones said the affair had made him "bitter in a sense," but he was later able to use that bitterness to help others.

"He will be remembered for his fight…and the fact that he came out and spent the rest of his life… working with those that were perhaps convicted unjustly," Jones told CTV’s News Channel.

During his fight to be freed, a group of Canadians befriended Carter and helped to keep his case in the spotlight.

Leon Friedman, who served as the chief appellant lawyer for Carter, said he'll remember the former boxer for the unwavering determination he showed in the face of his enormous legal battle.

"Right from the start, he showed the same determination in fighting his case as he did in fighting people in the ring,"

Friedman told CTV News Channel Sunday. "He was determined, insistent and very convincing on the fact that he was innocent."

Carter's story has been widely detailed in news stories, books, music and film.

His 1974 autobiography, "The Sixteenth Round," helped raise awareness for his case. One year later, Bob Dylan wrote the song "Hurricane," inspired by the boxer’s plight.

Most famously, Carter's story was the subject of the 1999 Norman Jewison movie "The Hurricane," which featured Denzel Washington in the lead role.

When he moved to Toronto, Carter served as executive director of the Association in Defence of the Wrongly Convicted from 1993 to 2004.

He eventually left the association after board members refused to condemn the appointment of prosecutor Susan MacLean to the Ontario Court of Justice. MacLean was a member of the prosecution in the case of Guy Paul Morin, the Ontario man who spent more than 10 years in prison for the death of Christine Jessop before DNA evidence helped clear his name.

Despite his split with the association, Carter never stopped speaking out for those he believed had been wrongly convicted.

In February 2014, Carter wrote an opinion article in the New York Daily News calling for a review of the case of David McCallum, a man who Carter said has been wrongfully imprisoned for 28 years.

In the article, Carter revealed that he was on his death bed and said the request for a review was his "final wish."

The article ends with the former fighter reflecting on his own life and his hopes for the future.

"If I find a heaven after this life, I’ll be quite surprised. In my own years on this planet, though, I lived in hell for the first 49 years, and have been in heaven for the past 28 years," he said.

"To live in a world where truth matters and justice, however late, really happens, that world would be heaven enough for us all."

Carter's family members, including two daughters, all live in the U.S., Artis said.

The former boxer's personal artifacts, including his writing and photos, will be donated to a Boston university.

With files from The Canadian Press