Irwin grew up in Indianapolis, the third youngest of four children, and began racing quarter-midgets before second grade. He graduated from Lawrence North High School in 1988, balancing varsity soccer with his driving career. Between 1988 and 1991 he earned his SCCA competition license and competed in the GT1 category in a turbocharged Buick Grand National, also racing in the IMSA American Challenge series.
Irwin then moved to USAC open-wheel competition, beginning in 1991. He accumulated seven career USAC Sprint Car Series wins and was the series Rookie of the Year in 1993. In 1994, he was USAC Silver Crown Series Rookie of the Year and finished second in the 1995 USAC standings. His crowning achievement in open-wheel was the 1996 USAC National Midget Series championship. His rapid development drew comparisons to Jeff Gordon, and many in the open-wheel community regarded him as one of the most promising talents of his generation.
Irwin made his Craftsman Truck Series debut in 1996 at Phoenix International Raceway, driving the No. 26 Ford for MB Motorsports. In just his second start at Richmond, driving the No. 62 Raybestos Ford for Liberty Racing, he won the pole and finished fifth. Moving to a full-time schedule in 1997 with Liberty Racing's No. 98 Ford, Irwin won twice, collected seven top-fives and ten top-tens, finished tenth in the final standings, and claimed Truck Series Rookie of the Year honors.
Irwin also made his Winston Cup Series debut in 1997 with David Blair Motorsports at Richmond, qualifying on the outside pole and finishing eighth after leading twelve laps — an astonishing debut performance. He ran three more races with Blair that season, qualifying no worse than eleventh.
In 1998, Irwin stepped up to a full-time Cup ride with Robert Yates Racing in the storied No. 28 car, replacing Ernie Irvan. He won the 1998 Winston Cup Rookie of the Year award despite a 28th-place finish in the points standings. The season included one pole, one top-five, and four top-ten finishes. In 1999, he improved to 19th in points with two poles, two top-fives, and six top-tens. He also made five Busch Series starts that year, posting two fifth-place finishes.
For the 2000 season, Felix Sabates tabbed Irwin to replace Joe Nemechek in Team SABCO's No. 42 Chevrolet. Through seventeen races, he had one top-ten — a fourth at Talladega Superspeedway. He also ran nine Busch Series starts for the team.
One of the defining storylines of Irwin's Cup career was his running feud with Tony Stewart, a former rival from their USAC days. At the NAPA Autocare 500 at Martinsville, Stewart wrecked Irwin twice in turn four; Irwin retaliated by spinning Stewart in turn one on the restart. Stewart exited his wrecked car, clapped sarcastically at Irwin, threw his gloves, and attempted to climb into Irwin's moving car under caution — one of NASCAR's most memorable confrontations.
During practice for the thatlook.com 300 at New Hampshire Motor Speedway on July 7, 2000, Irwin's car slammed head-on into the wall, flipped onto its side, and slid before rolling onto its roof. He died instantly of a basilar skull fracture at the age of thirty. The crash was blamed on a stuck throttle — the same cause as Adam Petty's fatal accident at nearly the same spot on the same track just eight weeks earlier in May 2000.
Tony Stewart, the man who had been Irwin's fiercest rival, won the race that Sunday and donated the trophy to Irwin's parents.
The dual tragedies of Petty and Irwin at New Hampshire, followed by Tony Roper's death at Texas in October 2000 and Dale Earnhardt's at Daytona in February 2001, ultimately compelled NASCAR to mandate the HANS device in October 2001. SAFER barriers were adopted at oval tracks hosting NASCAR and IndyCar races in 2002.
Irwin's parents founded the Kenny Irwin Jr. Foundation and the Dare to Dream Camp for underprivileged children in New Castle, Indiana. The 2000 Brickyard 400, held on what would have been Irwin's 31st birthday, was dedicated in his memory.