The idea for the original Ironman Triathlon arose during the awards ceremony for the 1977 Oahu Perimeter Relay (a running race for 5-person teams). Among the participants were numerous representatives of both the Mid-Pacific Road Runners and the Waikiki Swim Club, whose members had long been debating which athletes were more fit, runners or swimmers. Ironman Triathlon was the first major competition to extend the distance to an extreme endurance event. The first Ironman Triathlon was held on February 18, 1978 in Honolulu, Hawaii, repeated in 1979 and 1980.

Although thousands of athletes worldwide compete at an Ironman event each year, the vast majority aim simply to just finish the course if they are first timers, or set a PR (personal record) time if they've raced this distance before. Only very talented athletes realistically compete for a spot in Hawaii, and just finishing an Ironman race is often the highlight of many triathletes' career. Athletes with disabilities now compete in the event in the physically challenged category, and are required to meet the same cutoff times as able bodied competitors. Australian John MacLean was the first physically challenged athlete to complete the event.

People completing such an event within the strict event time cutoffs are agreed to be recognized as "Ironmen": the plural "Ironmans" refers to multiples of "Ironman" as a short form of "Ironman Triathlon". In the triathlon community an Ironman is someone who has completed a race of the appropriate distance, whether or not it falls under the aegis of WTC.