Gary Winnick, a telecommunications tycoon who rose from junk-bond salesman to found a massive venture that laid 100,000 miles of undersea fiber-optic cable worldwide, only to see it flame out in a multibillion-dollar bankruptcy, died Nov. 4 at his home in Los Angeles. He was 76.

His death was confirmed by a family death notice, but no cause was reported.

Mr. Winnick, a disciple of Michael Milken, the purveyor of risky high-yield bonds known as junk bonds, founded the Global Crossing telecommunications company in 1997 to take advantage of the internet boom and the deregulation of telephone and cable television markets.

The aim was to build the world’s first privately financed fiber-optic network connecting North America, Europe, Latin America and Asia. The company ultimately was able to deliver services to hundreds of cities in dozens of countries.

In April 1999, Forbes magazine featured Mr. Winnick on its cover with the headline: “Getting Rich at the Speed of Light.”

At the peak of Global Crossing’s stock price that year, the company was valued at $47 billion – more than McDonald’s or PepsiCo – making Mr. Winnick a billionaire faster than anyone else in U.S. history at the time, including Microsoft co-founder Bill Gates, and prompting a local business magazine to declare Mr. Winnick the wealthiest person in Los Angeles.

He purchased the 8.5-acre Casa Encantada estate in Bel-Air, featuring a 40,000-square-foot, 60-room Georgian mansion built in the 1930s and once owned by hotel magnate Conrad Hilton. He paid $94 million for the estate in 2000, then a record for a U.S. home, and poured tens of millions of dollars more into renovating it.

But in an epic rise-and-fall story, the company never actually turned a profit, and when the dot-com bubble burst, Global Crossing proved unable to find enough customers for the capacity it had built. In 2002, it filed for bankruptcy protection, unable to repay debts totaling about $12 billion.

The company’s collapse despite $22.4 billion in assets ranked as the fourth-largest bankruptcy in history, according to BankruptcyData.com. Even in the peak year of its stock value, Global Crossing still lost a staggering $1.4 billion.

Among the investors wiped out by the company’s crash were many of its own employees. At the same time, however, Mr. Winnick avoided serious damage to his bottom line, having sold at least $735 million worth of Global Crossing stock over the years, much of it after company executives had warned internally of financial problems that were not publicly disclosed.

Lawsuits followed, charging the company with financial manipulation. In 2004, Mr. Winnick contributed $30 million to a settlement with two pension funds totaling $325 million. He also came under investigation by the Securities and Exchange Commission, which eventually fined three company executives. Mr. Winnick was not one of them, however.

Testifying before a congressional committee in 2002, Mr. Winnick said the bankruptcy was “not a result of fraud but of a catastrophe that befell an entire industry sector.” This was “certainly not an acceptable excuse,” he added. “It’s an explanation.”

In 2011, Global Crossing was sold to Level 3 Communications for $3 billion.

Gary Winnick was born in Manhattan on Oct. 13, 1947, and was raised in Roslyn, N.Y., on Long Island. His father sold restaurant supplies, and his mother was a homemaker and interior decorator.

Mr. Winnick, who was 18 when his father died, worked various jobs to put himself through school and, in 1969, received a bachelor’s degree in business from C.W. Post College (now LIU Post) on Long Island.

He was a furniture salesman for a time before joining a Wall Street company that came to be known as Drexel Burnham Lambert. He sold poorly rated bonds under Milken, who pioneered their use to finance tech start-ups and leveraged buyouts.

In 1972, Mr. Winnick married Karen Binkoff, who went on to write children’s books. In addition to his wife, he is survived by three sons, Adam, Alexander and Matthew; a sister; and eight grandchildren.

In 1975, Mr. Winnick moved his family to Los Angeles to work in Drexel’s Beverly Hills office as Milken’s chief assistant, making millions of dollars in the junk-bond business. He left the company in 1985 to start his own private equity firm.

Five years later, Drexel collapsed and Milken pleaded guilty to securities and tax fraud. Milken was sentenced to 10 years in prison, fined $600 million and permanently barred from the securities industry by the SEC. But he was released after less than two years for cooperating with authorities and for good behavior. At Mr. Winnick’s urging, President Donald Trump pardoned Milken in 2020.

Drawn to the promise of fiber-optic cable in servicing the burgeoning internet, Mr. Winnick founded Global Crossing in March 1997 after learning much of what he felt he needed to know about fiber optics by watching a video. He put $20 million of his own money into the venture, took it public in 1998 and raised $20 billion for it from investors over three years.

In the highflying days before the downturn, he earned a reputation as a leading philanthropist, donating more than $100 million to causes including the Los Angeles Zoo, the Special Olympics, the Simon Wiesenthal Center and the U.S. Holocaust Memorial Museum.

(In June, Mr. Winnick and his wife put their Bel-Air estate on the market for $250 million, which would be a record for a publicly listed home sale in the United States.)

In his October 2002 testimony before a congressional investigative subcommittee, Mr. Winnick expressed “profound sorrow” over the impact of Global Crossing’s crash, especially on its employees, but he denied engaging in fraud, insider trading or other “chicanery.” He argued that “neither our bankruptcy nor the global telecom meltdown that precipitated it is unique to our company.”

Mr. Winnick added: “This is not about money. This is about people. . . . Yes, I made a lot of money. But when I went into this venture, building a cable across the Atlantic, I had no contemplation that this thing would turn out to be what it was. I am both proud and I am saddened by it.”