Modern triathlon

The first known swim/bike/run triathlon was held at Mission Bay, San Diego, California on September 25, 1974. The race was conceived and directed by Jack Johnstone and Don Shanahan, members of the San Diego Track Club, and was sponsored by the track club. 46 brave participants entered this never-before held event. It is well-documented and was not based on the French events.
Ironman
The first modern long-distance triathlon event was the Hawaiian Ironman Triathlon. It included a 2.4 mile (3.86 km; 77 lap) swim, a 112 mi (180.2 km) bike ride, and a 26.2 mi (42.195 km) run. It was conceived 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. On this occasion, U.S. Navy Commander John Collins pointed out that a recent article in Sports Illustrated magazine had declared that Eddy Merckx, the great Belgian cyclist, had the highest recorded "maximum oxygen uptake" of any athlete ever measured, so perhaps cyclists were more fit than anyone. Collins and his wife, Judy, had taken part in the triathlons staged in 1974 and 1975 by the San Diego Track Club in and around Mission Bay, California, as well as the Optimist Sports Fiesta Triathlon in Coronado, California in 1975.
A number of the other military athletes in attendance were also familiar with the San Diego races, so they understood the concept when Collins suggested that the debate should be settled through a race combining the three existing long-distance competitions already on the island: the Waikiki Roughwater Swim (2.4 mi/3.862 km), the Around-Oahu Bike Race (115 miles (185 km); originally a two-day event) and the Honolulu Marathon (26.219 mi/42.195 km). No one present had ever done the bike race so they did not realize it was a two-day, not one-day, event. Collins calculated that, by shaving 3 miles (5 km) off the course and riding counter-clockwise around the island, the bike leg could start at the finish of the Waikiki Rough Water and end at the Aloha Tower, the traditional start of the Honolulu Marathon. Prior to racing, each athlete received three sheets of paper listing a few rules and a course description. Handwritten on the last page was this exhortation:
“
Swim 2.4 miles! Bike 112 miles! Run 26.2 miles! Brag for the rest of your life!
”

— Commander Collins, (1978)
With a nod to a local runner who was notorious for his demanding workouts, Collins said:
“
Whoever finishes first, we'll call him the Ironman.
”

— Commander Collins, (1978)
Of the fifteen men to start off in the early morning on February 18, 1978, twelve completed the race and the world's first Ironman, Gordon Haller, completed it in 11 hours, 46 minutes, and 58 seconds.