The Western States 100 is sponsored by Montrail and is one of the four 100 mile races that comprise the Grand Slam of Ultrarunning, which also includes the Vermont 100 Mile Endurance Run, the Wasatch Front 100 Mile Endurance Run in Utah, and the Leadville Trail 100 in Colorado. Western States is one of the valid qualifying events for the Hardrock 100.
The WS Endurance Run was first completed in 1974 by Gordy Ainsleigh. Ainsleigh had finished the Tevis Cup Trail Ride in 1971 and 1972 on horseback, but in 1973 his new horse was pulled with lameness at the 29-mile checkpoint. In 1974, his horse turned up lame and, according to legend, Ainsleigh declared that he would run the entire course on foot. An incoherent Ainsleigh finished the event 23 hours and 47 minutes later.
In 1975, Ron Kelley ran the Tevis Cup course along with the horses, and completed about 97 miles (157 km) of the course before dropping out. In 1976, Ken 'Cowman' Shirk became the second runner to complete the course along with the horses, with friend Ainsleigh pacing him the last 25 miles.
Fourteen runners signed up for the first official Western States Endurance Run in 1977, and started along with the horses in the Trail Ride. Eleven of the 14 had dropped out or were pulled by the midpoint that year. Of the three remaining runners, only Andy Gonzales finished in the 24 hour time limit set for the horses. The other two, Peter Mattei and Ralph Paffenbarger, finished in 28 hours and 36 minutes (unofficially), leading to the establishment of the 30-hour bronze buckle time limit for runners.
The following year, 1978, 63 runners competed in the first Western States Endurance Run. The race was held on a separate date, independent of the Tevis Cup Trail Ride.
In 1984, the Granite Chief Wilderness was created under the provisions of the 1964 Wilderness Act, and about four miles (6 km) of the trail were within the new boundaries. The wilderness designation would normally mean that the Forest Service would not be able to allow organized events in the area. In 1988, however, the Endurance Run (and the original Trail Ride) was finally given Congressional permission to continue, but with the number of runners limited to 369, the size of the 1984 field. As the event grew in notoriety, a lottery system was created to allocate the available positions. Each year since, a limited number of entries have been awarded to selected winners of certain other runs, or to top ten finishers of the preceding Western States race. After the few select entries are awarded, a lottery is then held to fill the field from a pool of qualified applicants. The Forest Service allows race management to use a five-year running average of 369 actual starters, and historically some fifteen percent of lottery winners do not report to the starting line, so some 425 runners are typically notified as being eligible.
Today the race is one of the premier ultrarunning events in the world. As of 2006, Tim Twietmeyer has completed the race a record 25 times, all in under 24 hours. With his 2005 victory, Scott Jurek has won seven consecutive races and set a new course record of 15 hours, 36 minutes, 27 seconds in 2004.
In 2008 the run was canceled due to bad air quality and smoke from an unprecedented number of wildfires.