Miller began training in mixed martial arts in 2005 with his brother, fellow UFC veteran Dan Miller. The Millers grew up in Sparta and Jim competed in wrestling at Sparta High School, as well as at Virginia Tech. He currently holds a black belt in Brazilian jiu jitsu under Jamie Cruz.
Miller had his first professional fight, on November 19, 2005, and in New Jersey, against Eddie Fyvie at Reality Fighting 10. Miller earned a unanimous decision. Miller later fought Muhsin Cobbrey in New Jersey for the Reality Fighting Lightweight Championship. In his first title defense, Miller took on future UFC Lightweight Champion Frankie Edgar at Reality Fighting 14 in New Jersey. Miller secured a tight guillotine late in the final round but Edgar fought through the choke to earn the unanimous decision. In the weeks prior to Reality Fighting 14, Miller began training at AMA Fight Club in Whippany.
Miller debuted with Cage Fury in a lightweight championship bout on April 13, 2007 in New Jersey on the same card as his brother for the first time since Reality Fighting XII and both were victorious. Miller defended his title for the first time, on an event in Atlantic City which featured the much-hyped fight between Kimbo Slice and Ray Mercer.
Jim signed with the UFC in July 2008 and made his debut at UFC 89 where he defeated David Baron via rear naked choke. Miller next competed against Matt Wiman at Fight for the Troops and won via unanimous decision. At UFC 100, Miller won a unanimous decision versus Mac Danzig. At UFC 103,Miller won after Steve Lopez injured his shoulder. At UFC 108, Miller submitted Duane Ludwig via armbar. At UFC 111 in Newark, Miller was awarded the unanimous decision against Mark Bocek. At Fight Night 22 Miller fought Gleison Tibau winning via unanimous decision. At UFC 124, Miller defeated Charles Oliveira via a first round kneebar and earned the Submission of the Night bonus. Miller next defeated Kamal Shalorus at UFC 128 in Newark via TKO. At UFC on Versus 5, Miller lost the fight via unanimous decision to Benson Henderson. At UFC on FX1, Miller submitted Melvin Guillard with a rear naked choke. Miller next faced Nate Diaz in Newark, in the main event of UFC on Fox. He lost the fight via submission–for the first time in his professional career–in the second round due to a guillotine choke. Miller faced Joe Lauzon at UFC 155 and won the fight via unanimous decision, with the performance earning Fight of the Night honors. The win also gave Miller the most all-time wins in UFC Lightweight history. Miller faced Pat Healy at UFC 159 in Newark and lost by submission in the third round. This fight earned Miller his third Fight of the Night bonus. Healy, however, subsequently tested positive for a banned substance and the result of the fight was overturned to a No Contest.
Jim lives in Lafayette with his wife and two children.