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.