Hallmann was born in Gdynia, Poland on August 25, 1987. He is the first professional athlete from his family. He had shown interest in martial arts at a very young age, when he had started training karate at the age of six. His other athletic pursuits included football and capoeira. He had started training in mixed martial arts in 2008, and made his professional debut one year later. He trains at Mighty Bulls Gdynia under Grzegorz Jakubowski. Hallmann graduated from Akademia Marynarki Wojennej and is currently an officer of the Navy, with the rank of Secondary Lieutenant.
Hallmann had made his professional debut at "Beast of the East" event in his hometown, Gdynia, on November 14, 2009, when he defeated Krzysztof Adaszak. After three consecutive wins, he suffered his first career defeat against a German fighter, Christian Eckerlin. Following that loss, Hallmann collected nine consecutive wins at various European events. He won Casino Fight Night 2011 tournament in Erfurt, as well as World Freefight Challenge and Celtic Gladiator lightweight championship titles.
Ultimate Fighting Championship
Following his series of wins on European circuit, Hallmann had signed a four-fight deal with the Ultimate Fighting Championship Hallmann first competed on the card, which took place on September 4, 2013 in Brazil. He faced Brazilian fighter Francisco Trinaldo and scored a comeback win by kimura in the second round, after getting repeatedly hurt by kicks to the body in the first round. This performance had earned him a $50,000 Submission of the Night bonus award. Hallmann's second fight in the promotion took place at on October 26, 2013. He had faced Al Iaquinta and, after three back-and-forth rounds, he had lost by unanimous decision. In his highest profile fight with the promotion to date, Hallmann faced Yves Edwards at UFC Fight Night 42 on June 7, 2014. He won the fight via rear-naked choke submission in the third round. The win also earned Hallmann his first Performance of the Night bonus award. Hallman next faced Gleison Tibau on September 13, 2014 at UFC Fight Night 51. He lost the back-and-forth fight via split decision. Despite the loss on the scorecards, Hallman was awarded a bonus for Fight of the Night. However, Hallman tested positive during a post fight drug screening for drostanolone. In turn, Hallmann was suspended for nine months and his bonus award was rescinded. Hallman faced promotional newcomer Magomed Mustafaev on June 20, 2015 at UFC Fight Night 69. He lost the fight via TKO in the second round, after the referee stopped the contest on the advice of the doctor due to a large cut over Hallman's left eye. Hallman faced Alex Oliveira on November 7, 2015 at UFC Fight Night 77. He lost the back-and-forth bout via knockout in the third round and was subsequently released from the promotion.
