Julianna graduated in 2007 from Mt. Spokane High School. She then attended Spokane Community College.
Peña made her professional MMA debut in May 2009. She won four consecutive fights but suffered her first defeat in April 2012 to future fellow The Ultimate Fighter 18 cast member Sarah Moras in a 140-pound catchweight bout. The fight took place just two months and one week after Peña was involved in an accident with a drunk driver while walking in downtown Spokane in which she was knocked unconscious and suffered a broken nose. Ten months later she returned to professional competition in a fight in the flyweight division, a move down from the bantamweight division, losing by unanimous decision.
In August 2013, it was announced that Peña was one of the fighters selected to be on The Ultimate Fighter: Team Rousey vs. Team Tate.
Peña faced Gina Mazany in the elimination fight to get into the house. She controlled the fight from early on, winning a clear unanimous decision victory after two rounds.
During the first tournament fight in the house, Peña faced veteran and top-ten ranked Shayna Baszler. Ronda Rousey selected this match up between the two first female picks. The highly ranked and more experienced Baszler was widely considered the favorite. After a back and forth first round in which Peña landed effective boxing combinations and escaped several of Baszler's submission attempts, Peña came out hard in the second round, connecting with powerful knee strikes that staggered Baszler. The fight went to the ground, where Peña was able to achieve back mount and won via rear naked choke for perhaps the biggest upset victory of the season.
In the semifinals, Peña faced off against Sarah Moras. The two fought previously in a professional bout in 2012, with Peña losing after suffering an injury due to an armbar which forced the doctor to stop the bout between the second and third rounds. Peña won the fight via guillotine choke in the second round to avenge the loss.
Peña faced Jessica Rakoczy in the finals on November 30, 2013 at The Ultimate Fighter 18 Finale. She won the bout via TKO in the final seconds of the first round.
Peña was expected to face Jessica Andrade at UFC 171 on March 15, 2014. However, Peña pulled out of the bout after suffering an injury to her right knee. She suffered the injury while grappling in training, ultimately damaging, among other aspects, her ACL, MCL, LCL and meniscus. Despite the severity of the injury, doctors have assured Peña that her right knee will return to full strength following surgery and rehabilitation. The injury kept Peña out of action for the rest of 2014.
Peña returned to face Milana Dudieva on April 4, 2015 at UFC Fight Night 63 She won the fight via TKO in the first round. The win also earned Peña her first Performance of the Night bonus award.
Peña next faced Jessica Eye on October 3, 2015 at UFC 192. She won the fight by unanimous decision.
Peña faced former title challenger Cat Zingano at UFC 200 on July 9, 2016. She won the fight by unanimous decision.
Peña faced Valentina Shevchenko in a possible title eliminator in the main event at UFC on Fox: Shevchenko vs. Peña on January 28, 2017. She lost the fight via armbar submission in the second round.
On Sunday, December 20, 2015 Peña was arrested in Spokane, Washington. Charged with 2 counts of Assault, "An alleged victim and two witnesses told officers that Peña and Josh Gow, her training partner on the Sikjitsu fight team, attempted to enter Zola's bar after being involved in a street fight. Witnesses said Gow's face was covered in blood from the brawl and the two wanted to use the bathroom to get Gow cleaned up. Bar employees told Peña and Gow that the establishment was closed and they could not come in, which prompted Peña to become "irate," according to one witness. Witnesses said that Peña kicked a bar employee in the groin and then, while outside the business, kicked the owner in the groin twice. Gow did not get physical with anyone, witnesses said, and was described as being clearly drunk. The two attempted to enter Zola's again, witnesses said, but were locked out. Gow then rubbed his bloody face on the bar's front window, according to witnesses."
Peña stated to the Spokane Police Department Sgt. Eric Kannberg, ″that they had been involved in a fight with 3 people on the street, Gow had suffered a facial injury and she wanted to take him to Zola's to clean up because she knew the owner. Peña said that employees at Zola's didn't let them in because Gow was bleeding, according to Kannberg. Peña indicated that they had then left, Kannberg said. He wrote that he asked Peña if anything happened and she said no.″
UFC vice president of public relations Dave Sholler has stated, "that the promotion is "aware of the situation" and "in the process of gathering more info."
A December court hearing for Peña was moved to January 19 in the Spokane County District Court. A judge later granted a stipulation order of continuance in her case such that if Peña had no other incidents for a year, the case would be dismissed.
Peña was featured in the award-winning mixed martial arts documentary Fight Life, the film is directed by James Z. Feng and released in 2013.Ultimate Fighting Championship
The Ultimate Fighter 18 Tournament Winner
Performance of the Night (One time) vs. Milana Dudieva
2013 Comeback of the Year