Paul Allen, the co-founder of Microsoft with Bill Gates, is dead. He died at 65. He had battled several diseases before giving up the ghost.

They were teenage computer geeks, bespectacled kids from Seattle who taught themselves programming from a Teletype terminal, learned the basics of business from Fortune magazine and dreamed of “a computer in every home and on every desk.”

The Washington Post reports that Paul Allen was the self-described “idea man,” the shy son of librarians. Bill Gates was the business-oriented partner who brought the ideas to life. And in 1975, when Mr. Allen was 22 and Gates was 19, the friends formed a company that became known as Microsoft and unleashed a personal-computer revolution that made both men fabulously wealthy.

Mr. Allen left the company after only eight years, amid a bout with Hodgkin’s disease and a deteriorating friendship with Gates. But he remained a powerful force in technology and philanthropy for decades, investing his billions in an eclectic array of businesses and charitable efforts while acquiring sports teams, discovering World War II shipwrecks and backing aerospace ventures that drew on his childhood fascination with ad­ven­ture stories and science fiction.

He was 65 when he died Oct. 15 in Seattle. The cause was complications of non-Hodgkin’s lymphoma, according to a statement from his family. Mr. Allen had battled the disease in 2009 and announced earlier this month that his lymphoma had returned.

When Mr. Allen and Gates founded their company, computers were bulky and expensive. Microprocessors had been invented just a few years earlier, and most monitors showed nothing but green or white characters on a black screen.

Technology companies were primarily interested in hardware — developing computers that were faster, stronger and smaller than anything that came before. But Mr. Allen and Gates, before most of their peers, realized that the programs a computer ran were just as consequential as the chips and wiring inside the machine. By the late 1990s, Microsoft operating systems would run on nearly 90 percent of personal computers in the United States.

Mr. Allen had left the company by the time Microsoft unveiled its Windows operating system and before it released ubiquitous programs such as the text-editing software Microsoft Word. But he helped oversee the development of groundbreaking products such as the early operating system MS-DOS, which launched Microsoft to national prominence through a partnership with IBM, and took credit for devising the two-button mouse and the company’s name, short for microprocessors and software. Microsoft’s success earned him and Gates tens of billions of dollars, and by 1992, a Wall Street research firm estimated that at least 2,200 Microsoft employees — nearly one-fifth of the company’s workforce — had become millionaires. The company’s co-founders, business journalist Brent Schlender wrote in Fortune magazine three years later, had created “more wealth than any business partners in the history of American capitalism.”

Gates, who served as the company’s chief executive until 2000, has a fortune of about $90 billion, second only to Amazon.com founder and Washington Post owner Jeffrey P. Bezos. Mr. Allen, who retained a sizable stake in Microsoft after leaving the company, is personally worth $21.7 billion, Forbes reported in March.

Both men insisted that they worked together on nearly every project in the company’s early years, to the point where it was all but impossible to distinguish authorship. “Our great string of successes had married my vision to his unmatched aptitude for business,” Mr. Allen wrote in a 2011 memoir, “Idea Man.”

But as Microsoft took off in the 1980s and ’90s, Mr. Allen encountered mixed success as an investor, leading critics to question whether he was not an “accidental zillionaire” whose wealth was “a lucky trick of time and place,” as Wired magazine wrote in 1994.

Through Vulcan, an investment company he named for the Roman god of fire and creation, he pursued a vision he dubbed the “wired world,” in which cable, satellite TV and the Internet played increasingly central roles in popular culture and commerce. But while Mr. Allen was correct in anticipating an era in which news stories were read on a screen, not solely in print, he struggled to reap the financial returns.

He leveraged his Microsoft holdings to acquire Charter Communications for $4.5 billion in 1998, but the company filed for bankruptcy protection in 2009, resulting in an estimated $7 billion loss for Mr. Allen. Skeptics also pointed to his 25 percent stake in America Online, which he sold for $140 million in 1994after unsuccessfully trying to convince the company to sell him more shares. At the height of the tech bubble, Reuters reported in 2013, “those shares would have been worth more than $40 billion.”

Despite the occasional downturn, his Microsoft stock holdings formed the bedrock of an investment portfolio that sometimes left him in shock, marveling at his good fortune. “At some point there is so much money, what do you do with it all?” he told the Los Angeles Times in 1995, explaining that he had just invested half a billion dollars into the newly formed Hollywood studio DreamWorks SKG.

Mr. Allen’s billions enabled him to play guitar with Stevie Wonder in a jam session on his yacht, hold a masked ball in Venice, build a $100 million pop music museum and acquire sports teams near his hometown of Seattle, including the Portland Trail Blazers, Seattle Seahawks and a share of the Seattle Sounders soccer team.

His 414-foot yacht, named Octopus, cost $200 million and featured a recording studio, helipads and two submarines, according to Forbes. Using his yacht and submersibles, he located World War II shipwrecks including the Musashi, a Japanese battleship, and the Indianapolis, a U.S. Navy cruiser that sank in shark-infested waters in 1945.

He also funded SpaceShipOne, which became the first private aircraft to put a civilian in suborbital space, and licensed its technology to Virgin Galactic, the space effort founded by Richard Branson. In 2011, Mr. Allen announced the creation of Stratolaunch Systems, a project to develop a new aircraft — considered the largest plane ever flown — that is supposed to launch spaceships into orbit.