Millen began his career in the New Zealand Rally Championship, winning the championship title in 1975, 1976, and 1977 driving Mazda RX3s. He moved to the United States in 1978 to pursue rally racing there with a Mazda rotary-powered RX7, winning the North American Race and Rally Championship in 1979 and again in 1980. He claimed the SCCA PRO Rally title in 1981 and a second PRO Rally series championship in 1985, followed by back-to-back SCCA National Rally Championships in 1987 and 1988.
In 1989, while winning his third straight SCCA National Rally Championship, Millen also took the FIA Asia-Pacific Rally Championship and won the GTU division at the 24 Hours of Daytona alongside co-drivers Al Bacon and Bob Reed. That same year he won the Class C division at the Pikes Peak International Hill Climb, beginning his long association with the mountain event.
Millen competed in selected World Rally Championship rounds from 1977 to 1992, with his most active period coming between 1989 and 1992 in a Mazda 323 AWD co-driven by Tony Sircombe. His best WRC result was second place at the 1989 Rothmans Rally of New Zealand, his home event.
Millen developed a reputation as one of the most skilled hillclimbers in the world across the early 1990s, winning the Open Class at Pikes Peak in 1991 and Showroom Class in 1992 before moving to the open rally division in 1993. In 1994, driving an all-wheel-drive Toyota Celica, he shattered the existing Pikes Peak record by 40 seconds, completing the 156-turn, 12.42-mile climb in 10 minutes 4.06 seconds. The record stood as the fastest time on the mountain's unpaved surface; after the course was partially paved, Nobuhiro Tajima recorded a faster overall time in 2007, but Millen's dirt-era mark remained a benchmark. He went on to record the fastest overall Pikes Peak time in 1996, 1997, 1998, and 1999 as well, though he was unable to break the 10-minute barrier that had become his personal goal.
Millen competed in the Mickey Thompson Stadium Series indoor off-road series from 1991, winning two events in 1992 and then claiming the Grand National Sport Truck series championship in 1992, 1993, and 1994 — the only driver in the series' 12-year history to win three consecutive championships. He received American Auto Racing Writers and Broadcasters Association All-American First Team recognition in 1992 and 1994.
In 1997 Millen won the South Africa Stadium series championship. He competed in the Race to the Sky hillclimb in New Zealand from 1998, finishing second in both 1998 and 1999 and winning the Unlimited Class in 2001 and 2002.
Millen also competed in off-road desert racing, contesting numerous editions of the Baja 1000 alongside his sons. In 2006 the team, with Ryan Millen as one of the drivers, finished second in class by 33 seconds after 34 hours of racing in what was described as a photo finish.
In 2007 Millen won the Transsyberia rally, covering 7,100 kilometres from Moscow to Ulan Bator in a Porsche Cayenne S Transsyberia as the fastest of 26 competing crews alongside co-driver Richard Kelsey.
He also competed in Formula D drifting during the mid-2000s.
Millen founded Rod Millen Motorsports, later renamed MillenWorks, which originally prepared and built his race cars before expanding into vehicle development, high-performance parts, and military technology projects. The company reflects the engineering dimension of Millen's motorsport career alongside his driving achievements.
Millen was inducted into the Pikes Peak Hill Climb Museum Hall of Fame in 2016. His family's combined motorsport involvement — spanning four members across multiple disciplines and multiple decades — represents one of the most extensive racing dynasties in the history of New Zealand motorsport.