Fell, who won a silver medal in the modern pentathlon at the 2008 Olympic Games in Beijing, claimed a top 10 age category placing in Chattanooga, Tennesee, in her new sport.
The 34-year-old was 10th in the 30-34 age group in her first world level ironman event.
She clocked an overall time of five hours, three minutes and 46 seconds.
Fell completed the swim in 29.35 minutes, finished the bike ride in 2:50.16 and then ran the half marathon in 1:37.24.
Her overall time was nearly 25 minutes quicker than she had done at the Ironman 70.3 Exmoor event in June, where she won her category and came third women overall.
Earlier this year Fell completed a full ironman race, finishing second in her category at the Standard Bank African Championships.
She clocked a time for that of 10:46.46, which has qualified for October’s World Ironman Championships in Hawaii.