The Manhattan U.S. attorney’s office also sought the extradition of Mr. McAfee in a separate criminal case.

“John was and will always be remembered as a fighter,” said Nishay K. Sanan, an attorney representing Mr. McAfee in U.S. criminal proceedings. “He tried to love this country but the U.S. Government made his existence impossible.”

Mr. Sanan said the court ruling Wednesday would be appealed in the next few days.

Spanish authorities will investigate the cause and circumstances of Mr. McAfee’s death, according to a government official. In December, a court rejected a request by Mr. McAfee that he be freed, saying he was a flight risk and that remaining in the jail near Barcelona didn’t pose a risk to his health.

Mr. McAfee founded the company that still bears his name in 1987, but he sold his stake in the 1990s for more than $100 million.

“Although John McAfee founded the company, he has not been associated with our company in any capacity for over 25 years,” a McAfee Corp. spokeswoman said via email Wednesday. “That said, our thoughts go to his family and those close to him.”

In the mid-2000s, Mr. McAfee gave up the corporate world and spent millions of dollars building a series of airstrips in the southwestern U.S., where he indulged his passion for “aerotrekking,” or low-altitude flyover exploration of desert terrain

In recent years, he had focused on cryptocurrencies, which he promoted with an outlaw image and a reputation for brash pronouncements. In 2017, Mr. McAfee in a tweet predicted that bitcoin would be valued at $500,000 within three years, a prediction that failed to come to pass.

Over the years, he had several run-ins with the law. While he lived in Belize, police sought to question him after his neighbor was found shot to death in 2012. He fled the country to Guatemala but was later deported to the U.S. after being found to have entered the country illegally. Mr. McAfee maintained his innocence on the matter.

Mr. McAfee earned millions promoting cryptocurrencies, charging speaking fees and providing consulting services, but failed to pay taxes on this income, federal prosecutors say. Authorities accused him of failing to file tax returns between 2014 and 2018 and avoiding taxes by naming other people as the owners of his assets in cryptocurrency accounts, real estate and even a yacht.

In March, federal prosecutors in New York charged Mr. McAfee with fraud saying he deceptively promoted cryptocurrencies via his Twitter account. Mr. McAfee and his associates made more than $2 million via a “pump-and-dump” scheme, where they promoted a dozen little-known cryptocurrencies on Twitter, then dumped the assets once their value had increased, prosecutors said.

In a statement posted on a website last year, Mr. McAfee said he hadn’t filed a tax return since 2010 and that he previously contacted the IRS about his plans. In a March tweet, Mr. McAfee said he was being used as a scapegoat in a broader U.S. crackdown against cryptocurrencies.

In one of his last public messages on Twitter, sent last week to his 1 million followers, Mr. McAfee claimed that his cryptocurrency assets had “dissolved” and the government had seized his remaining assets.

“My friends evaporated through fear of association,” the tweet said. “I have nothing. Yet, I regret nothing.”

Mr. Sanan said the court ruling Wednesday was going to be appealed in the next few days and that the New York extradition was still pending in a lower court in Spain.

Mr. McAfee founded the security company that still bears his name in 1987, but he sold his stake in the business in the 1990s and had not been involved in the company’s operations for decades.

—Giovanni Legorano contributed to this article.

Write to Bowdeya Tweh at Bowdeya.Tweh@wsj.com and Robert McMillan at Robert.Mcmillan@wsj.com