Nelson died of cardiovascular failure at his home in Kensington, Md., a Washington suburb, said Bill Christofferson, Nelson's biographer and a family spokesman.
"He died peacefully. His wife was with him," Christofferson said.
Thirty-five years after the first Earth Day, April 22 is still a day on which many people plant trees, clean up trash and lobby for a clean environment.
A conservationist years before it became fashionable, Nelson was recognized as one of the world's foremost environmental leaders. Then-President Clinton presented Nelson with a Presidential Medal of Freedom in 1995 for his environmental efforts.
"As the father of Earth Day, he is the grandfather of all that grew out of that event: the Environmental Protection Act, the Clean Air Act, the Clean Water Act, the Safe Drinking Water Act," read the proclamation from Clinton.
"Gaylord's contributions in the fields of conservation reform and environmental improvement are a living memorial to him," Melvin Laird, a nine-term congressman from Wisconsin and secretary of defense in the Nixon administration, said in a statement before the death was announced.
Nelson entered public life in 1948 as a Wisconsin state senator from Dane County, a position he held for 10 years. In 1958, Nelson became only the second Democrat during the 20th century to be elected governor of Wisconsin.
While in office, Nelson used a penny-a-pack tax on cigarettes to pay for the Outdoor Recreation Acquisition Program in 1961. The program allowed Wisconsin to buy hundreds of thousands of acres of parkland, wetlands and other open space.
After two two-year terms, Nelson was elected in 1962 to the U.S. Senate, unseating 78-year-old incumbent Republican Alexander Wiley.
In his three terms, he championed conservation policies, including legislation to preserve the 2,100-mile Appalachian Trail and create a national hiking system.
"He was just an incredible person: humble, funny, proud of his roots in Clear Lake, Wisconsin, and never changed by the power and pomp of the offices that he held," Gov. Jim Doyle said in a statement Sunday.
Nelson's most recognized effort, however, was Earth Day, which he started as an environmental demonstration based on the anti-war teach-ins of the Vietnam War.
"It suddenly occurred to me, why not have a nationwide teach-in on the environment," Nelson said. He announced his idea at a speech in Seattle in September 1969, and it "took off like gangbusters."
The first Earth Day, April 22, 1970, attracted an estimated 20 million people. Tens of thousands of people filled New York's Fifth Avenue, Congress adjourned so members could speak across the nation, and at least 2,000 colleges marked the occasion.
Nelson once said Earth Day worked because "it organized itself. The idea was out there and everybody grabbed it. I wanted a demonstration by so many people that politicians would say, 'Holy cow, people care about this.' That's just what Earth Day did."
In 1972, Sen. George McGovern, the Democratic presidential nominee, sought out Nelson as a potential running-mate. Nelson said no.
"Behind his humor and behind the sort of rough-cut, down-to-earth manner, there was always a person of sober conviction," McGovern said later.
Nelson continued to represent Wisconsin in the Senate until he was narrowly defeated in 1980 by Robert W. Kasten Jr., one of a raft of Republicans swept into office with Ronald Reagan.
He joined the Washington-based Wilderness Society and served as its full-time legal counselor. William H. Meadows, the group's president, called Nelson the "founding father of the modern environmental community."
In the Wilderness Society, Nelson more and more focused his attention on the world's quickly multiplying population. When he was born in 1916, the world's population was about 1.8 billion, and it grew to nearly 6 billion in 1999.
"The wealth of the nation is air, water, soil, forest, scenic beauty, wildlife habitat. Take that away and all that's left is a wasteland," he said in a June 1999 address to the Wisconsin Legislature.