Marker on the trail near Mount Sugarloaf in Maine commemorating its completion.
The trail was conceived by
Benton MacKaye, a forester who wrote his original plan—called "An Appalachian Trail, A Project in Regional Planning"
—shortly after the death of his wife in 1921. MacKaye's idea detailed a grand trail that would connect a series of farms and wilderness work/study camps for city-dwellers. In 1922, at the suggestion of Major
William A. Welch, director of the
Palisades Interstate Park Commission, his idea was publicized by
Raymond H. Torrey with a story in the
New York Evening Post under a full-page banner headline reading "A Great Trail from Maine to Georgia!" The idea was quickly adopted by the new
Palisades Interstate Park Trail Conference as their main project.
On October 7, 1923, the first section of the trail, from
Bear Mountain west through
Harriman State Park to
Arden, New York, was opened. MacKaye then called for a two-day Appalachian Trail conference to be held in March 1925 in Washington, D.C. This meeting inspired the formation of the Appalachian Trail Conference (now called the
Appalachian Trail Conservancy) (ATC).
A retired judge named
Arthur Perkins and his younger associate
Myron Avery took up the cause. In 1929, Perkins, who was also a member of the
Connecticut Forest and Park Association and its Blue Blazed Trails committee, found
Ned Anderson, a farmer in
Sherman, Connecticut, who took on the task of mapping and blazing the
Connecticut leg of the trail (1929–1933). It ran from
Dog Tail Corners in
Webatuck, New York, which borders
Kent, Connecticut, at Ashley Falls, 50 miles (80 km) through the northwest corner of the state, up to
Bear Mountain at the
 (A portion of the Connecticut trail has since been rerouted (1979–1983) to be more scenic, adhering less to highways and more to wilderness, and includes a Ned K. Anderson Memorial Bridge.)
Anderson's efforts helped spark renewed interest in the trail, and Avery (who led the charge after Perkins’ death in 1932) was able to bring other states on board. Upon taking over the ATC, Avery adopted the more practical goal of building a simple hiking trail. He and MacKaye clashed over the ATC's response to a major commercial development along the trail's path; MacKaye left the organization, while Avery was willing to simply reroute the trail. Avery reigned as Chairman of the ATC from 1932 to 1952 (he died that same year).
Avery became the first to walk the trail end-to-end, though not as a
thru-hike, in 1936. In August 1937, the trail was completed to
Sugarloaf Mountain in Maine, and the ATC shifted its focus toward protecting the trail lands and mapping the trail for hikers.
Many of the trail's present highlights were not part of the trail in 1937:
Roan Mountain, North Carolina and Tennessee; the
Mount Rogers high country, including
Grayson Highlands, Virginia; the
Pochuck Creek swamp, New Jersey; Nuclear Lake, New York; Thundering Falls, Vermont; and
Saddleback Mountain, Maine. Except for places where the
Civilian Conservation Corps was brought in (mostly in
Shenandoah National Park, the
Great Smoky Mountains, and Maine), the original trail often climbed straight up and down mountains, creating rough hiking conditions and a treadway prone to severe
erosion. The ATC's trail crews and volunteer trail-maintaining clubs have relocated or rehabilitated miles of trail since that time.
In 1936, A 121-day Maine to Georgia veteran's group funded and supported thru-hike was reported to have been completed, with all but three miles of the new trail cleared and blazed, by six
Boy Scouts from
New York City and their guides.
 The completed thru-hike was much later recorded and accepted by the
Appalachian Long Distance Hikers Association.
In 1938, the trail sustained major damage from a hurricane that went through the New England area. This happened right before the start of World War II and many of the people working on the trail were called to active duty.
Earl Shaffer of
York, Pennsylvania, brought a great deal of attention to the project by publicizing the first claimed thru-hike. The claim was later criticized for the hike's omission of significant portions due to short-cuts and car rides.
 Shaffer later claimed the first north-to-south thru-hike, the first to claim to do so in each direction.
 Chester Dziengielewski was later to be named the first south bound thru-hiker.
 In 1998, Shaffer, nearly 80 years old, hiked the trail, making him the oldest person to claim a completed thru-hike.
 The first solo woman to complete the hike was 67-year old Emma Gatewood who completed the northbound trek in 1955, taking 146 days. She repeated the achievement two years later.
In the 1960s, the ATC made progress toward protecting the trail from development, thanks to efforts of politicians and officials. The
National Trails System Act of 1968 designated the
Pacific Crest Trail and Appalachian Trail as the first national scenic trails and paved the way for a series of
National Scenic Trails within the
National Park and National Forest systems.
 Trail volunteers worked with the
National Park Service to map a permanent route for the trail, and by 1971 a permanent route had been marked (though minor changes continue to this day). By the close of the 20th century, the Park Service had completed the purchase of all but a few miles of the trail's span.