Oliver Turner, an English Channel swimmer and a 3-time Manhattan Island Marathon Swim finisher, founded the Horsetooth Open Water Swim in 1999.  While he was a graduate student at Colorado State University, he came up with the idea of a long distance swim that would challenge the swimmers and also be a fundraiser for the Wingshadow Foundation. His vision had the swimmers navigate the length of the reservoir (~10 km) where in alternating years it would be swum either north to south or south to north. The first year, there were 15 brave souls that swam the reservoir north to south.

A four-year dam repair and strengthening project began in 2000 that virtually drained the reservoir. This caused the swim to be temporarily moved to nearby Carter Lake in 2000 where a 4.8-mile out-and-back course was implemented.

In 2001, an alternate out-and-back 10 km course was identified in Horsetooth Reservoir.  Starting about midway, at the Sail and Saddle Club, swimmers went to the north end and back.

In its drained state,  Horsetooth Reservoir resembled two bodies of water with a channel between them that the swimmers needed to swim through.  In 2002, a weird situation occurred where the southernmost section was  16°C (60°F) while the northernmost  section was 21°C (70°F). The swimming from cold water into warm water and then back into cold water proved to be unique and particularly challenging to quite a few swimmers.

In 2003 the 2.4 and 1.2 mile swims were added. By 2004, Horsetooth Reservoir dam rework was complete and the swim was able to return to the original format of swimming the full length of the lake. The number of participants grew and with the addition of the shorter races, the direction of the swim changed.   Swimmers started at the south end and swam  north to finish in Satanka Cove.

In 2006, several exciting things happened with the race. First, the Horsetooth Swim 10K was awarded the U.S. Masters Swimming Long Distance Open Water Championship for the 6+ mile distance.  It also was the first year of youth (age group )(1000 yard, 250 yard) swims.

In 2007, with the closing of the Wingshadow Foundation, the beneficiary changed to Team Fort Collins whose mission is promoting healthy lifestyles through the prevention of substance abuse.

In 2011, Larimer County Parks completed a beautiful pavilion at the South Bay beach area. This provided a perfect, beautiful venue for hosting the shorter races and the finish line for the 10 km marathon swim.  The 10 km swim changed back to north –  south swim. The side effect of this change is the South Bay beach area is short of the full length of the lake by about 500 m. Several approaches have been made to address this.

In 2011, the 10 km marathon swimmers swam past the finish line about 250 mm and then back in to add the distance. In 2012, the water level was extremely low due to a severe drought such that there was not enough lake past the finish line to make the full 10 km distance. The decision was made to just swim point-to-point the length of what was left of the lake, resulting in a distance of 9.4 km (5.85 miles). In 2013 the water level returned to a more normal level and the decision was made to direct the swimmers into a cove near the start to add distance to make a full 10 km marathon swim.