[8] With a successful high school career, Dillashaw received a full-ride scholarship to wrestle for Cal State Fullerton Titans.

In April 2007, he placed sixth in the University Greco Wrestling Championships in Akron, Ohio, at 132.25 pounds.

[10] After graduating from California State University, Fullerton, in 2009 with a Bachelor of Science in kinesiology,[11] Dillashaw contemplated turning to MMA.

Dillashaw began training with Team Alpha Male, and collected a 2–0 amateur record before turning pro.

Dillashaw defeated Roland Delorme via rear naked choke submission in the preliminary round and moved onto the semi-finals.

[16] Dillashaw officially made his UFC debut on December 3, 2011, in Las Vegas, Nevada, at The Ultimate Fighter 14 Finale.

[18] Dillashaw dominated Watson using his superior wrestling to control him on the ground, while nearly ending the fight several times with multiple submission attempts.

[25] Dillashaw faced Hugo Viana on April 20, 2013, at UFC on Fox 7, replacing an injured Francisco Rivera.

[27] Dillashaw was briefly linked to a bout with Raphael Assunção on September 4, 2013, at UFC Fight Night 28.

[41] However, the day of the weigh-ins, Barão had to be admitted to the hospital as a result of his attempts to cut weight and was replaced by Joe Soto who was already scheduled to compete in a fight on the event's preliminary card.

[48][49] Dillashaw faced former champion Dominick Cruz on January 17, 2016, at UFC Fight Night 81[50] He lost the bout and the title via split decision.

[60] Dillashaw won the fight via knockout in the second round to regain the UFC Bantamweight Championship.

[68] On March 20, 2019, Dillashaw announced that he would be voluntarily relinquishing the UFC bantamweight championship after USADA and the New York State Athletic Commission (NYSAC) found adverse findings following his bout against Henry Cejudo.

[69][70][62][71] On April 9, it was announced Dilllashaw had been suspended for two years by USADA (an additional suspension to what was given by the NYSAC) due to testing positive for recombinant human erythropoietin (EPO) prior to the Henry Cejudo bout.

[73][74] On April 12, Dillashaw broke his silence over the drug test failure and UFC suspension with a post on Instagram, acknowledging that he had used a banned substance and stated that the decision was his alone, and that what he "really [felt] bad about" was the bad light he brought upon his coaches, family and teammates by association.

[75] He also said his use of EPO was limited to the Cejudo fight and said that USADA had retested all of the samples from his prior drug tests to confirm this.

[76] After the two-year hiatus due to his USADA suspension, Dillashaw was scheduled to face Cory Sandhagen on May 8, 2021, at UFC on ESPN 24.

[78] The pair was rescheduled and served as the main headliner for UFC on ESPN: Sandhagen vs. Dillashaw on July 24, 2021.

[80] The decision was seen as somewhat controversial, with many fans, fighters, and media members expressing their belief that Sandhagen won the bout.

[83] Dillashaw, who was hampered by a dislocated shoulder from early on in the bout,[84] lost the fight via technical knockout in round two.

[85][86] In his post-fight speech, Dillashaw admitted that he had come into the fight with an ongoing shoulder problem, saying "I probably dislocated it 20 times in training camp.

[89] In October 2015, Dillashaw left Team Alpha Male and moved his camp to Denver, Colorado, to train with his head coach, former UFC fighter Duane Ludwig.

Accusations included forcing promising Ultimate Fighter winner, Chris Holdsworth (6-0), to retire at the age of 27 due to concussions sustained from Dillashaw using illegal knees on a downed opponent during practice.