Van Lierde's international career started in 1990 when he came fourth in the World Olympic Distance Triathlon Championships. In the full-length Ironman triathlon, which involves swimming 3.8 km, cycling 180 km and running a 42.2 km marathon, he was ranked among the first ten in the European Championships three times between 1990 and 1995. 1995 was a decisive year for Luc Van Lierde, when he became World Vice Champion in the triathlon event and came second in the European Olympic Distance Triathlon Championships.
In 1996, coached by Mario Huys, he won the European Championships and came second in the World Championships in the Olympic Distance Triathlon. He won the Nice Triathlon and became World Long Course Triathlon Champion. Luc Van Lierde became the first European ever to win the Ironman World Championship, beating the existing record by three minutes.
Luc Van Lierde went on to clock the fastest Ironman Triathlon ever in 1997, doing 7:50:27 (0:44, 4:28, 2:36, plus transition) in Ironman Europe. Absent from the Ironman 1998, after undergoing an operation, he nevertheless won the Flemish Sports Personality of the Year trophy. In 1999, he once again won the Ironman World Championship in Hawaii, outrunning the second placed competitor by six minutes. During that same year, he received the Giant of Flanders trophy awarded by the two Flanders section of the Association of Professional Journalists.