According to Official Guinness Records,
The earliest modern triathlon was held by the San Diego Track and Field Club (USA) on 24 September 1974 in Mission Bay, California, USA. The initial event consisted of a 5.3-mile (8.5km) run, a 5-mile (8.05km) cycle race and 600 yards (548m) of ocean swimming. Today, standard "Olympic" distances for a triathlon are 10km (6.2 mi) run, 40km (24.8 mi) bike, and 1500m (1,640 yds) swim.
For a complete list of 1974 records, please visit 1974 Guinness Records in United States.