The aim of this case study was to report on the performance outcomes and subjective assessments of long-term low-carbohydrate, high-fat (LCHF) diet in a world-class long-distance triathlete who had been suffering from gastrointestinal distress in Ironman competition. The lacto-ovo vegetarian athlete (age = 39 years; height = 179 cm; usual racing body mass = 75 kg; sum of seven skinfolds = 36 mm) changed his usual high carbohydrate (CHO) availability diet to an LCHF diet for 32 weeks (similar to 95% compliance). He participated in three professional races while on the LCHF diet, but acutely restored CHO availability by consuming CHO in the preevent meals and during the race as advised. The athlete had his worst-ever half-Ironman performance after 21 weeks on the LCHF diet (18th). After 24 weeks on LCHF, he had his second worst-ever Ironman performance (14th) and suffered his usual gastrointestinal symptoms. He did not finish his third race after 32 weeks on LCHF. He regained his usual performance level within 5 weeks back on a high CHO diet, finishing second and fourth in two Ironman events separated by just 3 weeks. Subjective psychological well-being was very negative while on the LCHF diet, with feelings of depression, irritability, and bad mood. In conclusion, this long-term (32 weeks) LCHF intervention did not solve the gastrointestinal problems that the athlete had been experiencing, it was associated with negative performance outcomes in both the half-Ironman and Ironman competitions, and it had a negative impact on the athlete's subjective well-being.