Thomson was born and raised in San Jose, California, growing up in the east side of the city. Being half-English and half-Mexican, Thomson faced discrimination and got into fights often. In middle school, he was involved in a brawl in which the principal was punched while trying to break up the incident, resulting in Thomson being placed in a Juvenile hall. Afterwards, Thomson went to live with his father in Idaho where he went to high school and competed in wrestling. Thomson continued wrestling at junior college powerhouse, North Idaho Community College, and also attended Stanford University but decided the school was not for him after only attending for a few weeks. His college wrestling record was 40-3. In 2000, Thomson was involved in an alcohol-fueled brawl while on a lake cruise in Idaho. In the melee Thomson choked one man so badly that he required CPR and as a result, Thomson was charged with felony aggravated assault, serving six months of a three-year sentence before being released for good behavior.

On November 21, 2008, Thomson was scheduled to make his first title defense in a rematch against Yves Edwards at Strikeforce: Destruction but had to pull out after tearing ligaments in his toe. Duane Ludwig was selected to take Thomson's place on the card. On April 11, 2009, Thomson was scheduled to make his first title defense in a rematch against Gilbert Melendez at Strikeforce's debut on the Showtime Network, but had to pull out after breaking his ankle while training on April 1, 2009. Rodrigo Damm was selected to take Thomson's place on the card.

On December 19, 2009 at Strikeforce: Evolution, Thomson fought Gilbert Melendez for the Lightweight Championship. Thomson would lose the fight by unanimous decision, but the fight would see both fighters put it all on the line in what would prove to be a very fast-paced and exciting fight.

Thomson's next fight was against Tatsuya Kawajiri under the DREAM banner at its end of year event Dynamite!! 2010. He lost the fight via unanimous decision.

Thomson once again faced Gilbert Melendez for the Strikeforce Lightweight Championship at Strikeforce: Barnett vs. Cormier on May 19, 2012. He lost the fight via split decision.

Thomson was expected to face current UFC Lightweight Champion Anthony Pettis on December 14, 2013 at UFC on Fox 9, replacing an injured TJ Grant, However, the bout was cancelled after Pettis pulled out of the bout citing a knee injury.

Thomson was expected to face Michael Johnson on July 26, 2014 at UFC on Fox 12. However, on July 11, Johnson pulled out of the bout due to injury and was replaced by Bobby Green. Thomson lost the bout to Green by split decision.

In his second fight for the promotion, Thomson faced Pablo Villaseca on December 4, 2015 at Bellator 147. He won the fight via TKO in the second round.