By the early 20th century, most of these forests had been cut down. Just north of the San Francisco Bay, one valley named Redwood Canyon remained uncut, mainly due to its relative inaccessibility.
This was noticed by U.S. Congressman William Kent. He and his wife, Elizabeth Thacher Kent, purchased 611 acres (247 ha) 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 (119 ha) 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. President Roosevelt agreed, writing back:
- MY DEAR MR. KENT: By George! you are right.
and, responding to some photographs of Muir Woods that Mr. Kent had sent him,
- Those are awfully good photos.
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 280 feet (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 40 feet (12 m) 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, damaging several other trees nearby. The closed trails have since been reconfigured and reopened.
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.
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.
The monument was listed on the National Register of Historic Places on January 9, 2008.