Lawford died of a heart attack Tuesday in Vancouver, Canada, his cousin, former U.S. Rep. Patrick J. Kennedy, told The Associated Press. He was 63.

Lawford was in Vancouver living with his girlfriend and working to open a recovery center. He had been doing hot yoga, which he did often, but the strain of it "must have been too much for him at that point," Kennedy said.

Lawford was the only son and oldest child of Patricia Kennedy — sister of John, Robert and Ted Kennedy — and Peter Lawford — the English actor and socialite who was a member of Frank Sinatra's "Rat Pack."

"I was given wealth, power and fame when I drew my first breath," Lawford wrote in his 2005 book, "Symptoms of Withdrawal: A Memoir of Snapshots and Redemption," the first of several books he wrote about his substance struggles.

He wrote that his parents got telegrams predicting big things for him from Bing Crosby and Dean Martin and said he once got a lesson in doing "The Twist" from Marilyn Monroe. The cover of his books shows him sitting poolside as a child with his uncle and soon-to-be-president John F. Kennedy looming behind him.

He spent his youth frolicking with Hollywood stars on one coast and rubbing shoulders with political stars on the other, living between libertine Los Angeles and the hyper-competitive Kennedy compound in Hyannis Port, Massachusetts, where he was a big-brother figure to John F. Kennedy Jr.

"You can't get much more fawned over than being a Kennedy male," Lawford wrote.

His life with drugs began with LSD while at boarding school at age 14. In the years before he had experienced the assassinations of his two uncles and his parents' divorce in 1966.

With heroin and other opioids as his substances of choice, Lawford leapt into deeper substance abuse in drug-heavy 1970s Hollywood, where his father also abused drugs and alcohol as his career faded. Peter Lawford died in 1984. Patricia Kennedy died in 2006.

In his memoir, Christopher Lawford told tales of mugging women for money, panhandling in Grand Central Station and getting arrested twice for drug possession before getting sober at 30.

"There are many days when I wish I could take back and use my youth more appropriately," Lawford told The Associated Press in 2005. "But all of that got me here. I can't ask for some of my life to be changed and still extract the understanding and the life that I have today."

Patrick Kennedy, the former congressman from Rhode Island whose father is Edward M. Kennedy, said his cousin "did something very difficult," airing family secrets and temporarily hurting his relationships within the Kennedy clan when he wrote his book.

"He had the courage to know that he had to find himself, and he wasn't going to be able to do it while holding on to the old family narrative," Kennedy said.

Lawford was "tormented by the fact" that for a time he was estranged from his sisters, Patrick Kennedy said.

"Over the years of recovery, he ended up reconciling with his sisters, happiest I ever saw him," Kennedy said.

His life's work became helping others recover — including his cousin.

"He was the absolute cornerstone to my sobriety, along with my wife," Patrick Kennedy said (the former politician had been addicted to drugs and alcohol). "He was the one who walked me through all the difficult days of that early period."

After his memoir, Lawford authored several more books on addiction and recovery, most recently 2015's "What Addicts Know."

He worked steadily as an actor, with moderate success. He had a small part in 2003's "Terminator 3: Rise of the Machines," made appearances on TV shows including "Frazier" and "The O.C." and had recurring roles on the soaps "All My Children" and "General Hospital," playing a senator in the latter.

He told the AP in 2005 that his famous dual identities both helped and hurt him in Hollywood.

"The names give you entree, absolutely, but it's a kind of a double-edged sword," he said. "People do pay attention to you, but nobody gets ahead in Hollywood unless they are really lucky or they deserve it."

He is survived by his sisters, Sydney, Victoria and Robin, and his children, David, Savannah and Matt.