Millen began his racing career in
hillclimbing in 1969. He then moved to the
Formula Ford series. He started competing in
rally cars in the mid 1970s. He switched to
off-road racing in the
Mickey Thompson Entertainment Group (MTEG) stadium series in the early 1980s. He won the 1986, 1988, and 1989 Mickey Thompson Off-Road Grand Prix Grand National Sports Truck Championships. He also won two
Indy Lights races in the series' inaugural 1986 season, that same year he also participated in the last round of the World Rally Championship in the
21st Olympus Rally driving for
Toyota Team Europe, accompanying the Swedish drivers
Björn Waldegård and
Lars-Erik Torph. Millen switched to the
IMSA Camel GT series in 1990. He was the
1990 24 Hours of Le Mans Rookie of the Year. He won three races and took five pole positions in 1990. He had seven poles and five wins in 1991. He also drove for
Allan Moffat Racing in the
1991 Tooheys 1000 at
Bathurst, partnering
Gary Brabham in a
Ford Sierra RS500 but the car did not finish the race. Millen expressed interest in returning but no offer was forthcoming with the Moffat team seeing the return of the car's builder Ruedi Eggenberger in 1992 along with his ace driver
Klaus Niedzwiedz. Millen won the 1992
IMSA GTS championship in a
Nissan 300ZX Twin
Turbo car. He won the
12 Hours of Sebring and had top five finishes in the first four events before his 1993 season ended with injuries at
Watkins Glen International. Millen returned in 1994 and won the IMSA GTS championship. He won the 1994
24 Hours of Daytona and
12 Hours of Sebring and was first in class at the
24 Hours of Le Mans. In 1995, Millen won the 12 Hours of Sebring. He was invited to the
International Race of Champions (IROC). He was leading the IMSA GTS championship, when he had a professional career-ending crash at
Road Atlanta. He had competed in three of four IROC races. Millen has the most all-time wins in IMSA GTS history with 20 victories. His 23 pole positions are also a series record. He has had several wins in occasional appearances in various series since his retirement. Millen also worked as a driver for commercials sometimes with
Rod and has appeared in a commercial for Honda, despite being under contract at the time to rival
Nissan. He is also a member of the
Screen Actors Guild as a stunt driver, which provided the healthcare he needed when he had his career ending crash. ==Business career==