He is the son of Rob Ridnour, a former high-school basketball coach and current coach of the Bellingham Slam. When Luke was a high-school sophomore, he was given the keys to the gym by his dad to practice at night. He was on two state title winning teams at Blaine High School, and was named a high school All-American by both McDonald's and Parade in 2000, his graduating year.
Ridnour went on to star at the University of Oregon, where he teamed with Luke Jackson to take the Ducks to the NCAA Men's Division I Basketball Tournament twice including the Elite 8 in 2002. He set the school season record for assists (218) and made a conference-record 62 consecutive free throws.
Ridnour left Oregon after his junior year, when he was Pac-10 player of the year, and was picked 14th in the 2003 NBA Draft by the Sonics. Ridnour played sparingly his rookie season, but became the starting point guard for the Sonics in the 2004-05 season. He participated in the 2005 All-Star weekend, playing in the Rookie Challenge and in the Skills Challenge.
Nate McMillan, despite giving substantial minutes to career backup PGs like Kevin Ollie and Antonio Daniels in place of Ridnour, is reported to have projected Ridnour as a future superstar of the NBA. He is widely compared to Steve Nash and John Stockton, although his statistics such as PER are actually inferior at roughly the 6,000-minute mark of his career when compared to those two at the same point. Ridnour considers Michael Jordan one of his biggest influences and personal heroes.