We investigated the timeline of performances in the three races of the ‘World Challenge Deca Iron Triathlon’, held in 2006, 2007 and 2009, where the athletes completed one Ironman triathlon daily on 10 consecutive days. The association of anthropometric characteristics such as body fat estimated using bioelectrical impedance analysis and previous experience in ultra-triathlon with race time was investigated using multiple linear regression analysis. Forty-nine athletes participated in these three races; 23 (47%) participants completed the race within 8,817 (1,322) min. Day 1 was the fastest with 762 (86) min; the slowest was Day 10 with 943 (167) min (P < 0.05). The time per Ironman increased during the race (P < 0.05). Body mass and fat mass decreased whereas lean body mass increased (P < 0.05). Race time was related to both the number of finished Triple Iron triathlons (P = 0.028) and the personal best time in a Triple Iron triathlon (P < 0.0001). We concluded that performance in a Deca Iron triathlon decreased throughout the competition, with the fastest race on Day 1 and the slowest on Day 10. The number of finished Triple Iron triathlons and the personal best time in a Triple Iron triathlon, but not anthropometry, were related to race time. To conclude, athletes need to have a high number of previously completed Triple Iron triathlons, as well as a fast personal best time in a Triple Iron triathlon, in order to finish a Deca Iron triathlon successfully.