Millen began racing in hillclimbing in New Zealand in 1969 and subsequently moved into Formula Ford. He expanded into rally competition in the mid-1970s. By the early 1980s he had relocated to the United States to compete in the Mickey Thompson Entertainment Group (MTEG) stadium off-road series, where he proved highly competitive. He won the Mickey Thompson Off-Road Grand Prix Grand National Sports Truck Championship in 1986, 1988, and 1989, demonstrating his broad range across different disciplines.
In the 1986 inaugural season of Indy Lights, Millen won two races. That same year he also participated in the final round of the World Rally Championship in the Olympus Rally, driving for Toyota Team Europe alongside Swedish works drivers Björn Waldegård and Lars-Erik Torph.
Millen switched to the IMSA Camel GT series in 1990 and immediately made an impression. He was named Rookie of the Year at the 1990 24 Hours of Le Mans, won three races, and took five pole positions in his debut season. He followed with seven poles and five wins in 1991, establishing himself as one of the leading protagonists in the series.
In 1992, driving a Nissan 300ZX Twin Turbo, Millen won the IMSA GTS championship and captured the 12 Hours of Sebring. His 1993 season was ended prematurely by injuries sustained in a crash at Watkins Glen International.
He returned for 1994 in emphatic fashion, winning the IMSA GTS championship for the second time. That season he won the 24 Hours of Daytona and the 12 Hours of Sebring, and also finished first in class at the 24 Hours of Le Mans — one of the most complete single-season records in American sports car racing.
Millen's dominance continued in 1995. He won the 12 Hours of Sebring for the third consecutive year and was leading the IMSA GTS championship when a serious crash at Road Atlanta ended his professional career. He had also been competing in that year's International Race of Champions (IROC), completing three of four rounds before his season was curtailed.
His career total of 20 wins and 23 pole positions in IMSA GTS remain all-time records for the class.
Millen founded Stillen in 1986, an aftermarket performance parts company specialising in Nissan vehicles and other performance cars. During the 1990s, Stillen produced customised high-performance Chevrolet Suburban SUVs and a range of modified Nissan special-edition vehicles. The company became a well-regarded name in the performance aftermarket industry.
Millen also pursued engineering and design work related to hillclimbing, contributing to car development for events including the Pikes Peak International Hillclimb and New Zealand's Race to the Sky at Queenstown.
As a Screen Actors Guild member and commercial stunt driver, Millen appeared in several automotive commercials, including one for Honda during his Nissan contract period.
Steve Millen comes from an accomplished motorsport family. His brother Rod Millen became a legend at the Pikes Peak International Hillclimb, renowned for his exploits in extreme hillclimbing events. Steve is the uncle of Rhys Millen and Ryan Millen, both of whom have pursued professional motorsport careers.
Steve Millen's record in IMSA GTS stands as one of the most impressive compiled by any driver in American sports car racing. His combination of raw speed — evidenced by 23 pole positions — and race-winning ability across consecutive championship campaigns placed him at the very summit of the discipline. His victories at Daytona, Sebring, and Le Mans across multiple years cement his place among the elite names in endurance sports car racing of the 1990s. The premature end to his career at Road Atlanta in 1995 denied him the opportunity to extend what was a historically dominant run.