Ian Crocker at the World Swimming Championships in 2005
Crocker is a specialist for short distances and butterfly appeared in the world rankings for the first time at the World Swimming Championships in 2001 when he won the silver medal in the 100 m butterfly. Previously, he had indeed already won at the 2000 Olympic Games in Sydney, a gold medal in the 4x100 m medley relay, but it was silver in a single competition, the breakthrough for him. In 2003, he was then world record holder and world champion in the 100 m butterfly and won silver over 50 meters butterfly.
At the 2004 Olympic Games in Athens, he was the big favorite over 100 m Butterfly, but the new star Michael Phelps had to concede defeat and won the silver medal. A gold medal from Athens, he again took the 4x100 m medley relay with home. With the 4x100 m freestyle relay, he won the bronze medal.
In Montreal at the World Swimming Championships in 2005, he was beaten, had been taken from him in the semifinals of his world record and this then in the final again in 22.96 s improved to the non-Olympic 50m Butterfly distance of South African Roland Schoeman Mark. But Crocker won the 100m butterfly in world record time and was also with the American 4x100 m medley relay world champion.
2008 Crocker qualified behind Michael Phelps at the Olympic Trials in 2008 over 100m butterfly for the 2008 Summer Olympic Games in Beijing where he finished fourth and finally announced end of his career.