SEATTLE — Paul G. Allen, who co-founded Microsoft with his childhood friend Bill Gates before becoming a billionaire philanthropist who invested in conservation, space travel and professional sports, died Monday in Seattle. He was 65.

He died from complications of non-Hodgkin’s lymphoma, his company Vulcan Inc. announced.

Microsoft CEO Satya Nadella called Allen’s contributions to the company, community and industry “indispensable.”

Two weeks ago, Allen announced that the non-Hodgkin’s lymphoma that he was treated for in 2009 had returned and he planned to fight it aggressively.

“My brother was a remarkable individual on every level,” Allen’s sister Jody Allen said in a statement. “Paul’s family and friends were blessed to experience his wit, warmth, his generosity and deep concern,” she added.

Allen, an avid sports fan, owned the Portland Trail Blazers and the Seattle Seahawks. He also was a part owner of the Seattle Sounders FC, a major league soccer team.

Allen and Gates met while attending a private school in north Seattle. The two would later drop out of college to pursue the future they envisioned: A computer in every home.

Gates so strongly believed it that he left Harvard University in his junior year to devote himself full time to his and Allen’s startup, originally called Micro-Soft. Allen spent two years at Washington State University before also dropping out.

They founded the company in Albuquerque, N.M., and their first product was a computer language for the Altair hobby-kit personal computer, giving hobbyists a basic way to program and operate the machine.

After Gates and Allen found some success selling their programming language, MS-Basic, the Seattle natives moved their business in 1979 to Bellevue, Wash., not far from its eventual home in Redmond.

Microsoft’s big break came in 1980, when IBM Corp. decided to move into personal computers and asked Microsoft to provide the operating system.

Gates and company didn’t invent the operating system. To meet IBM’s needs, they spent $50,000 to buy one known as QDOS from another programmer, Tim Paterson. Eventually the product refined by Microsoft — and renamed DOS, for Disk Operating System — became the core of IBM PCs and their clones, catapulting Microsoft into its dominant position in the PC industry.

The first versions of two classic Microsoft products, Microsoft Word and the Windows operating system, were released in the mid-1980s. By 1991, Microsoft’s operating systems were used by 93 percent of the world’s personal computers.

The Windows operating system is now used on most of the world’s desktop computers, and Word is the cornerstone of the company’s prevalent Office products.

Gates and Allen became billionaires when Microsoft was thrust onto the throne of technology.

With his sister Jody Allen in 1986, Paul Allen founded Vulcan, the investment firm that oversees his business and philanthropic efforts. He founded the Allen Institute for Brain Science and the aerospace firm Stratolaunch, which has built a colossal airplane designed to launch satellites into orbit. He has also backed research into nuclear-fusion power.