Rhys Millen is the son of Rod Millen, a Pikes Peak Hill Climb champion and prolific motorsport competitor who held the event's course record for thirteen years. The family connection to Pikes Peak gave Rhys an early familiarity with one of motorsport's most demanding events. Before turning his primary attention to Pikes Peak, Millen built a reputation in Formula Drift, winning the 2005 series championship — the second title in that series' short history at the time, following Samuel Hubinette's inaugural title. Millen recorded two Formula D wins: the 2004 season finale at Irwindale Speedway and the 2005 season opener at Wall Speedway in Wall Township, New Jersey. He competed in Formula D driving a Pontiac GTO for several seasons before transitioning to a Hyundai Genesis Coupe for the 2009 season.
In 2009, Millen partnered with Hyundai to contest Pikes Peak and select Redline Time Attack Series events with the Hyundai Genesis Coupe. That year, he set a new rear-wheel-drive record at Pikes Peak. In 2011 Millen drove a 500 bhp all-wheel-drive Hyundai Veloster in the US Rallycross Championship, and in the years following he competed in the Global Rallycross series.
Millen emerged as a frontrunner at Pikes Peak specifically in the electric vehicle division. In 2012, when the Pikes Peak course was fully paved for the first time in the event's history, Millen won the Time Attack Division overall, setting a new record in the process. His 2013 run with a time of 9 minutes 2.192 seconds placed him second overall behind Sébastien Loeb's record-breaking 8 minutes 13.878 seconds, which broke Millen's own previous record by more than 44 seconds as Loeb demolished the standing benchmark with the Peugeot 208 T16.
In 2015, Millen drove the Drive eO PP03 to first place overall, with Nobuhiro Tajima finishing second. In 2016, driving the Drive eO PP100, Millen again placed second overall. His 2015 electric car record of 9 minutes 7.222 seconds set a benchmark that was subsequently used as the target when Volkswagen Motorsport brought the all-electric I.D. R to Pikes Peak in 2018; the I.D. R's team stated a goal of beating Millen's 2016 record of 8 minutes 57.118 seconds for the electric class.
Beyond competitive driving, Millen has worked as a stunt driver for feature films including The Dukes of Hazzard, Mr. and Mrs. Smith, and The Fast and the Furious: Tokyo Drift. On 31 December 2008, he became the first person to execute a back-flip in an off-road truck.