A proud father. A brilliant friend. Lucky in love. Inherited wealth. And an opioid addict, dead at 54.

Four years ago, Matthew Mellon was living in an art-filled apartment in the Pierre hotel with his young second wife, Nicole Hanley. He had three young children, the eldest of whom lives with his first wife, Tamara Mellon, a founder of Jimmy Choo.

His marriage to Ms. Hanley was breaking up by 2015. The next year, he told the New York Post he had been taking 80 OxyContin pills a day, spending $100,000 a month on the habit.

And, last weekend, Mr. Mellon traveled by private plane to Cancun, Mexico. He was going to get a “touch-up treatment” at a rehab facility he had visited earlier this year, said Mr. Mellon’s stepfather, J. Reeve Bright — and had been sober for about 70 days.

He never made it there.

“He was supposed to check in Monday morning but we received word around 9 a.m. that he has passed away,” said Dr. Alberto Solà, the medical director of Clear Sky Recovery, an ibogaine treatment center. Mr. Mellon's family is awaiting results of an autopsy and toxicology tests being conducted by the Mexican government, Mr. Bright said.

Earlier in the week, Mr. Mellon visited with his parents in Delray Beach, Fla., where he was raised. “We had dinner with him on Saturday night, a joyful dinner and a very memorable dinner, more than we realized at the time,” Mr. Bright said. “He hugged and kissed his mother and he hugged and kissed me. He said, ‘I love you, Dad,’ and I said that I loved him. And 48 hours later he was dead.”

In Search of the New

Mr. Mellon was a member of one of the country’s oldest banking families: the great-great-great grandson of Judge Thomas Mellon, the Mellon Bank patriarch. Matthew Mellon’s father, Karl N. Mellon, died in 1983 at the age of 45, by suicide.

Matthew’s last name and social connections resulted in a lot of breathless press coverage of his wealth. It's unclear how much he actually inherited or earned, and what remained after many years of substance abuse. (Dominick Dunne said that Mr. Mellon received $25 million when he turned 21.)

Mr. Mellon, who had dyslexia, attended Phelps, a boarding school outside of Philadelphia. He was raised by his mother, Anne, and Mr. Bright; they married in 1975. Mr. Mellon received a bachelor’s degree from the Wharton School at the University of Pennsylvania in 1989.

According to his first wife, Tamara Mellon, he took his first trip to rehab while still in school. After college, he tried for careers in the music and fashion industries, marrying Ms. Mellon in 2000. The two met in a 12-step program.

Ms. Mellon installed her new husband as the creative director of Jimmy Choo’s collection of men’s shoes, but their relationship was turbulent in the office and at home. In 2003, Mr. Mellon started his own shoe company, Harrys of London. “When your wife makes $100 million during the course of your marriage, it’s quite a shocker,” he told W magazine in a 2007 interview. “I felt like my masculinity had been stripped from me. I was no longer the big man in the relationship. I feel like my balls are in a jar, like a Damien Hirst artwork on the mantelpiece. And here I am, ball-less.”

The divorce was acrimonious, and Mr. Mellon was tried in a British courtamid allegations he had tried to hack into his wife’s email. He was cleared of the charge. His lawyer argued in court the Mr. Mellon was too disorganized to have pulled off the crime. “His former wife spent most of her 90-minute testimony portraying Mellon, a former cocaine addict, as a loving but bumbling incompetent. She said he ‘missed planes like other people missed buses,’” the Telegraph reported at the trial’s conclusion. The exes eventually became friends.

Mr. Mellon remained interested in fashion for several years. He started another label, Degrees of Freedom, a luxury athleisure brand, with his fiancée Noelle Reno.

They never wed, but in 2010, Mr. Mellon married Ms. Hanley, herself a member of a wealthy family (her father was inducted into the Petroleum Hall of Fame in 2015). They had two children and together built a fashion line, Hanley Mellon, which was the subject of a New York Times story in 2014 that exposed the two to mockery. By 2015, the marriage was dissolving.

Mr. Mellon remained close to both his former wives. “His ex-wives are possibly even more devastated than his mother and I are,” Mr. Bright said. “We were laughing as we were looking at pictures earlier. There was a picture of Matthew standing between Tamara and Nicole and all of them are smiling. I don’t know how he did it!”

Most recently, Mr. Mellon had been spending time with Kick Kennedy, the daughter of Robert F. Kennedy, Jr. Ms. Kennedy, who had been working on her uncle Chris Kennedy’s bid for the Illinois governorship, declined to comment on the nature of their relationship. Mr. Mellon, once chairman of the New York Republican Party Finance Committee, donated more than $5,000 to Mr. Kennedy’s unsuccessful campaign. Mr. Mellon was also a Republican National Committee delegate for the 2012 presidential election.

The Crypto-Ambassador

Toward the end of his marriage to Ms. Hanley, he expanded his professional interests to bitcoin, the cryptocurrency.

“Matthew was very interested in the idea of the blockchain and what bitcoin could be very early on,” said David Marshack, who was an adviser to Coin.co, a bitcoin payment processor, along with Mr. Mellon. “He didn’t understand the underlying 0’s and 1’s of the technology but understood that the technology could make things cheaper, faster, more efficient.”

In 2014, Mr. Marshack and Mr. Mellon formed their own company, MellonDrexel, that allowed them to consult companies in the nascent cryptocurrency sector. Mr. Mellon was particularly taken with one of those companies, Ripple, for which he became a global ambassador, using his connections to market the company to banks and to his rich or famous friends. He took much of his compensation in its currency, called XRP.

“He risked everything on it and toward the end of last year, it exploded and made him an awful lot of money,” Mr. Marshack said. “It made a bunch of people second-guess their early criticisms. Though, to be fair, the criticisms were completely valid at the time they were made.”

In Treatment

Throughout his adult life, Mr. Mellon wrestled with drug addiction and was in and out of rehab. “Matthew has struggled,” his stepfather said, speaking of his son in the present tense. “There is no question that he has struggled. He has gone through some difficult things in his life. But he pushed and pushed and pushed himself.”

Friends described Mr. Mellon as an avid experimenter who wanted to be an early adopter of the new next thing — a good coffee, a good movie or a new apartment. That excitement, they said, may have lent itself to his addiction

“Matthew’s head was always in the clouds,” Mr. Marshack said. “But he was warm and generous and compassionate. The thing that made him excited about everything was the same thing that got him sick. It was part of him trying to do more, see more, experience more.”

By 2016, he had become involved in the opioid-addiction treatment called ibogaine, posting from Cancun, “#ibogaine Let me know if you ever hit a wall in life and you need help I will send you here for free on me!” Ibogaine is a plant-based substance banned in the U.S. that provokes hallucinations but also, proponents say, can decrease users’ cravings and symptoms of withdrawal.

He sought treatment at Clear Sky earlier this year.