Alzheimer's disease, according to one of his daughters, Diana Engelbart Mangan.
The mild-mannered Engelbart had audacious ideas. Long before Apple founder Steve Jobs became famous for his dramatic presentations, Engelbart dazzled the industry at a San Francisco computer conference in 1968.
Working from his house with a homemade modem, he used his lab's elaborate new online system to illustrate his ideas to the audience, while his staff linked in from the lab. It was the first public demonstration of the mouse and video teleconferencing, and it prompted a standing ovation.
"We will miss his genius, warmth and charm," said Curtis R. Carlson, the CEO of SRI International, where Engelbart used to work. "Doug's legacy is immense. Anyone in the world who uses a mouse or enjoys the productive benefits of a personal computer is indebted to him."
Back in the 1950s and '60s, when mainframe computers took up entire rooms and were fed data on punch cards, Engelbart already was envisioning a day when computers were far more intuitive to use.
One of the biggest advances was the mouse, which he developed in the 1960s and patented in 1970. The idea was way ahead of its time. The mouse didn't become commercially available until 1984, with the release of Apple's then-revolutionary Macintosh computer.
Engelbart's conceived the mouse so early in the evolution of computers that he and his colleagues didn't profit much from it. The technology passed into the public domain in 1987, preventing him from collecting royalties on the mouse when it was in its widest use. At least 1 billion have been sold since the mid-1980s.
Now, their usage is waning as people merely swipe their finger across a display screen.
"There are only a handful of people who were as influential," said Marc Weber, founder and curator of the Internet history program at the Computer History Museum, where Engelbart had been a fellow since 2005. "He had a complete vision of what computers could become at a very early stage."
Among Engelbart's other key developments in computing, along with his colleagues at SRI International and his own lab, the Augmentation Research Center, was the use of multiple windows. His lab also helped develop ARPANet, the government research network that led to the Internet.
Engelbart played down the importance of his inventions, stressing instead his vision of using collaboration over computers to solve the world's problems.
"Many of those firsts came right out of the staff's innovations - even had to be explained to me before I could understand them," he said in a biography written by his daughter.
In 1997, Engelbart won the most lucrative award for American inventors, the $500,000 Lemelson-MIT Prize. Three years later, President Bill Clinton bestowed Engelbart with the National Medal of Technology "for creating the foundations of personal computing."
Douglas Carl Engelbart was born Jan. 30, 1925, and studied electrical engineering, taking two years off during World War II to serve as a Navy electronics and radar technician in the Philippines. It was there that he read Vannevar Bush's "As We May Think" and was inspired by the idea of a machine that would aid human cognition.
Engelbart later earned his Ph.D. at University of California, Berkeley, but after joining the faculty, he was warned by a colleague that if he kept talking about his "wild ideas" he'd be an acting assistant professor forever. So he left for the Stanford Research Institute, now SRI International.
Engelbart is survived by his wife, Karen O'Leary Engelbart; his four children, Diana, Christina, Norman and Greda; and nine grandchildren.