The Western States 100 is one of the five 100-mile races that comprise the Grand Slam of Ultrarunning, which also includes the Old Dominion 100 in Virginia, the Vermont 100 Mile Endurance Run, the Wasatch Front 100 Mile Endurance Run in Utah, and the Leadville Trail 100 in Colorado.

The course was first attempted on foot by twenty soldiers from Fort Riley, Kansas during the 1972 Tevis Cup horse ride event.

Because the Tevis Cup imposed a 24-hour time limit, the soldiers started one day before the horses.

Sixteen runners signed up for the first official Western States Endurance Run in 1977, held in conjunction with the Tevis Cup.

Of the three remaining runners, only Andy Gonzales finished in the 24-hour time limit set for the horses.

[2] Later that year, the Western States Trail Foundation established the race's first Board of Directors.

The following year, 1978, the Western States Endurance Run was made independent of the Tevis Cup and took place in June, a month earlier than the horse ride.

The wilderness designation prevented the Forest Service from permitting organized events in the area.

In 1988, however, the Endurance Run (and the original Trail Ride) was given Congressional permission to continue, but with the number of runners limited to 369, the size of the 1984 field.

In 2008, the race was canceled due to bad air quality and smoke from an unprecedented number of wildfires.

The course then crosses the American River Canyon, ascends through Cool, and finishes at the Placer High School track in Auburn, California.

Approximately twenty miles in, runners continue down into Duncan Canyon before making a 1,500 foot ascent back up towards Robinson Flat.

Between miles 30 and 45, the trail drops to elevation 3,000 feet, at which point runners make the ascent to "Devil's Thumb".

Runners who complete the Western States Endurance Run in less than 24 hours are awarded a hand-made silver belt buckle.

Ann Trason holds the record for most women's victories, having won the race fourteen times.