The Monument is an old-growth coastal redwood forest. Due to its proximity to the Pacific Ocean, the forest is regularly shrouded in coastal fogs, contributing to a wet environment that encourages vigorous plant growth. The fog is also vital for the growth of the redwoods as they use moisture from the fog during the dry summer.
Before the logging industry came to California, there were an estimated 2 million acres (8,000 km²) of old growth forest containing redwoods growing in a narrow strip along the coast.
By the early 20th century, most of these forests had been cut down. Just north of the San Francisco Bay, one valley named Sequoia Canyon remained uncut, mainly due to its relative inaccessibility.
This wasn't unnoticed by U.S. Congressman William Kent. He and his wife, Elizabeth Thacher Kent purchased 611 acres (2.47 km²) of land from the Tamalpais Land and Water Company for $45,000 with the goal of protecting the redwoods and the mountain above them.
In 1907, a water company in nearby Sausalito planned to dam Redwood Creek, thereby flooding the valley. When Kent objected to the plan, the water company took him to court to attempt to force the project to move ahead. Kent sidestepped the water company's plot by donating 295 acres (1.2 km²) of the redwood forest to the Federal Government, thus bypassing the local courts.
On January 9, 1908, President Theodore Roosevelt declared the land a national monument, the first to be created from land donated by a private individual. The original suggested name of the Monument was the Kent monument but Kent insisted the Monument be named after naturalist John Muir, whose environmental campaigns helped to establish the national park system.
In December 1928, the Kent Memorial was erected at the Kent Tree in Fern Canyon. This tree — a Douglas fir, not a redwood ‐ was said to be Kent's favorite. Due to its height of (85 m) and location on a slope, the tree leaned towards the valley for more than 100 years. According to Muir Woods Storms in El Niño years of 1981 and 1982 caused the tree to tilt even more and took out the top of the tree. During the winter of 2002-03, many storms brought high winds to Muir Woods causing the tree to lean so much that a fissure developed in January 2003. This fissure grew larger as the tree slowly leaned more and more, forcing the closure of some trails. On March 18, 2003 at around 8:28 pm, the tree fell with a thunderous boom, damaging several other trees nearby. The closed trails have since been reconfigured and reopened.
In the spring of 1945, delegates from 50 countries met in San Francisco to draft and sign the United Nations Charter. President Franklin Delano Roosevelt died on April 12, 1945, shortly before he was to have opened the United Nations Conference. On May 19, the delegates held a commemorative ceremony in tribute to his memory in Muir Woods' Cathedral Grove, where a dedication plaque was placed in his honor.
In 1937, the Golden Gate Bridge was completed and park attendance tripled, reaching over 180,000. Muir Woods is one of the major tourist attractions of the San Francisco Bay Area, with 776,000 visitors in 2005.
The star attraction of the Muir Woods is the Coast Redwood (Sequoia sempervirens). These relatives of the Giant Sequoia are known for their height. While redwoods can grow to nearly (), the tallest tree in the Muir Woods is (). The average age of the redwoods in the Monument are between 500 and 800 years old with the oldest being at least 1,200 years old.
While overshadowed (and shaded) by their tall cousins, other tree species grow in the understory of the woods. Three of the most common are the California Bay Laurel, the Bigleaf Maple and the Tanoak. Each of these species has developed a unique adaptation to the low level of dappled sunlight that reached them through the redwoods growing overhead. The California Bay Laurel has a strong root system that allows the tree to lean towards openings in the canopy. The bigleaf maple, true to its name, has developed the largest leaf of any maple species allowing it to capture more of the dim light. The tanoak has a unique internal leaf structure that enables it to make effective use of the light that filters through the canopy.
It is occasionally possible to see Northern Spotted Owls or pileated woodpeckers in the forest. While decreasing in numbers elsewhere, the Spotted Owls appear to be thriving in the Monument and other evergreen forests in the area. A National Park Service monitoring project of the owls is ongoing within the Monument. The project has found that adult owls are finding mates, raising young to adulthood and the young are having new broods of their own.
There are 11 species of bats that call the Monument home, often time using hollows burned into the redwoods by past fires as a maternity colony.
Muir Woods, part of the Golden Gate National Recreation Area, is a park which caters to pedestrians. Hiking trails vary in the level of difficulty and distance. Picnicking, camping and pets are not permitted.
Bicycles are only allowed on fire roads.
Ranger-led walks on a number of different topics, including discussions on the watershed, wildflowers, and tidepools are held on weekends. Moonlight walks are held on nights with a full moon. Reservations are required for the moonlight walks.
Daily presentations are possible if staffing permits.
Special events are held for the summer and winter solstices.