Despite of the growth of ultra-endurance sports events (of duration >6 h) over the previous few decades, the age-related declines in ultra-endurance performance have drawn little attention. The aim of the study was to analyse the changes in participation and performance trends of older (>40 years of age) triathletes between 1986 and 2010 at the Hawaii Ironman triathlon consisting of 3.8 km swimming, 180 km cycling and 42 km running. Swimming, cycling, running and total times of the best male and female triathletes between 18 and 69 years of age who competed in the Hawaii Ironman triathlon were analysed. The relative participation of master triathletes increased during the 1986–2010 period, while the participation of triathletes younger than 40 years of age decreased. Linear regression showed that males older than 44 years and females older than 40 years significantly improved their performances in the three disciplines and in the total time taken to complete the race. Gender differences in total time performance significantly decreased in the same time period for all age groups between the 40–44 and 55–59 years ones. The reasons for these relative improvements of Ironman athlete performances in older age groups remain, however, unknown. Further studies investigating training regimes, competition experience or sociodemographic factors are needed to gain better insights into the phenomenon of increasing participation and improvement of ultra-endurance performance with advancing age.