Henry David Thoreau (1817-1862) - Author, poet, naturalist, surveyor, historian, transcendentalist, ...
  Known for his book Walden, a reflection upon simple living in natural surroundings.
John_Muir (1838-1914) - Naturalist, author, leader in the formation of the modern environmental movement.
Theodore Roosevelt (T.R.) (1858-1919) - 26th president or the United States 1901-1909
William O. Douglas (1898-1980) - Supreme Court Justice 1939-1975
Ansel Adams (1902-1984) - Photogropher and environmentalist
Rachel Carson (1907-1964) - Marine biologist and nature writer; Author of "Silent Spring"
William Edward Colby (1875-1964) - Lawyer, conservationist, and first Secretary of the Sierra Club.
David Brower (1912-2000) - First Executive Director of the Sierra Club 1952-1969
Dr. Edgar Wayburn (1906-) - Physician, 5 term president of the Sierra Club 1961-1969
David Suzuki (1936-) - Canadian science broadcaster and environmental activist.
Al Gore (1948-) - Vice President of the United States under Clinton 1993 to 2001.
Robert F. Kennedy Jr. (1954-) - Environmental lawyer

• William Kent - California Congresman who saved Muir Woods.
• Caroline Livermore - founded both the Marin Audubon Society and the Marin Conservation League.
• Phillip Burton (1926-1983) - United States House of Representatives 1964-1983
• Dr. Edgar Wayburn (1906-) - Physician, 5 term president of the Sierra Club 1961-1969
• Amy Meyer - Author, Environmentalist
• John Jacobs (1926-2002) - 2nd Executive Director of San Francisco Planning and Urban Renewal Agency (SPUR)
• Gordon Bennett - Marin County Environmentalist
• Dr. Marty Griffin was instrumental in creating Audubon Canyon Ranch and other wildlife sanctuaries along the county's coast and baylands.
• Phyllis Faber and Ellen Straus were prime movers in establishing the Marin Agricultural Land Trust in 1980.
• Mill Valley's Huey Johnson, founder of the Trust for Public Land and former resources secretary for California, continues to develop ideas and organizations that promote green plans for sustainability.
• Marin's Environmental Heritage at Terra-Marin.com

US Presidents

Other 19th Century Pioneeers:
Marsh, George Perkins (1801-1882). Diplomat, geographer, and author of "Man and Nature"
Powell, John Wesley (1834-1902). Geologist, ethnologist, explorer
Pinchot, Gifford (1865-1948). Forester, a founder of the conservation movement,
   First Chief of the United States Forest Service, 1905 , governor of Pennsylvania
Mather, Stephen Tyng (1867-1930) - Industrialist and conservationist.
   First director of the National Park Service, 1915.
Aldo Leopold (1887-1948) -  Ecologist, forester, and environmentalist
Olmsted, Frederick Law  Jr. (1890-1957) - Best known for his wildlife conservation efforts.
   Landscape architect - (Son of Frederick Law Olmsted, Sr. - "Father of landscape architecture")
John P. Saylor (1908-1973) - Republican member of the House of Representatives from Pennsylvania.
  He was dedicated to a number of environmental causes, including
  the Wilderness Act of 1964, and the National Wild and Scenic Rivers Act.
  He advanced a currently rare view of the natural environment as crucial to
  America's development and the culture of independence. He was dubbed "St. John"
  by environmental advocates for his dogged work on environmental issues.

"Man and Nature", by George Perkins Marsh, 1864
It is one of the first works to document the effects of human action on the environment and it helped to launch the modern conservation movement. Marsh argued that ancient Mediterranean civilizations collapsed through environmental degradation. Deforestation led to eroded soils that led to decreased soil productivity. Additionally, the same trends could be found occurring in the United States.

"Silent Spring", by Rachel Carson, 1962
Ranked with Thomas Paine's "Common Sense" and Harriet Beecher Stowe's "Uncle Tom's Cabin" for it's influence social change in American life.
Carson exposed the hazards of the pesticide DDT, eloquently questioned humanity's faith in technological progress and helped set the stage for the environmental movement.

Republicans:
In an era when conservative Republicans display outright hostility to environmentalism, it is interesting to look at their legacy.
The individuals credited with saving the most wilderness, Dr. Wayburn (100 million acres) and Theodore Roosevelt (230 million acres) were both republicans. Others include John Muir, Gifford Pinchot, John Saylor, Abraham Lincoln and even Richard Nixon who established the EPA.

Links:
Famous Conservationists
Environmentalism and Conservation Movement at wikipedia
American Environmentalists at DuckDuckGo.com
Living People Influenced by John Muir
Marin's Environmental Heritage at Terra-Marin.com