In 2004, he won his first series championship. Busch drives car #97, sponsored by Sharpie pens, Irwin Tools and Crown Royal for Roush Racing.Busch started his career in driving in the Craftsman Truck Series. He made a controversy when Jack Roush invited him to race in the (then) Winston Cup Series (later renamed when Nextel took over series sponsorship), without any experience in the "junior" Busch Grand National Series.

Busch began racing on the Winston Cup circuit in 2000, at the age of 21. He drove in 7 races, with no wins, top 5's, or top 10's, and finished 48th in the standings that year.Busch ran for rookie of the year honors in 2001, driving 35 of 36 races with no wins, although Kurt collected 3 top 5's and 6 top 10's that year. He also got a pole qualifying position that year at Darlington Raceway. Kurt finished 27th in the standings that year.

Busch almost won the championship in 2002, getting four wins, 12 top 5s and 20 top 10s, with one pole. He finished 3rd in the standings that year. He also collected $5,105,394.Busch had an up and down year in 2003, earning four wins, nine top 5s and 14 top 10s while finishing a disappointing 11th in the standings, although he collected US$5 million+ dollars again that year.

However, his season was marred by an ongoing feud with fellow driver Jimmy Spencer. After some bumping at a race at the Michigan International Speedway, Spencer bumped Busch with his car following the race and then walked over to Busch's car and punched him. Both drivers were fined and placed on probation for the rest of the year. The altercation harmed Busch's relationship with fans significantly. For various reasons, he remains an unpopular driver with many NASCAR fans, even in lieu of (and in some cases, because of) his championship win. He has since attempted to repair this image, but a tirade following his first-lap spin-out in 2005 at Darlington Raceway hindered these efforts. Despite his lack of fan appeal, most recognize him as one of the sport's greatest talents.

In 2004, Busch won the Nextel Cup by 8 points over Jimmie Johnson and 16 points over Jeff Gordon. In 2005, Kurt's brother Kyle Busch joined him on the Nextel Cup docket.As of October 16, 2005, he is 10th in the Nextel Cup standings.Busch will leave Roush Racing to join Penske Racing South starting in 2007. Busch had asked team owner Jack Roush to let him out of his contract at the end of the 2005 season, possibly to replace the retiring Rusty Wallace in the #2 Miller Lite Dodge Charger next season, but Roush refused. Busch also became engaged to girlfriend, Eva Bryan, while attending a race overseas in late July.In September 2005, his younger brother, Kyle Busch won his first race.