Mateusz Szczecinski was born on October 13, 1994, in Poland. Initially drawn to football (soccer) during his childhood, Szczecinski showcased his athletic versatility early on. At a local School Sports Club, he notably excelled in sprinting, particularly the 100-meter dash. Influenced by Poland's strong combat sports culture, he transitioned into martial arts at age 12, beginning his journey with boxing.

At age 13, Szczecinski started training Brazilian Jiu-Jitsu under Sylwester Dziekanowski at Gold Team. His rapid progression and dedication earned him his black belt from Dziekanowski on December 21, 2017. Szczecinski later moved to Academia Gorila in Koszalin, Poland, continuing his evolution into an elite-level competitor.

Mateusz Szczecinski is widely recognized for his aggressive, submission-focused grappling style, particularly in no-gi formats. Over 90% of his victories come via submission, with the straight ankle lock standing out as his signature technique. Szczecinski's relentless pursuit of submissions, combined with his precise leg attacks and positional dominance, has established him as one of the most dynamic and exciting grapplers of his generation.

In June 2023, Szczecinski captured the Polaris Pro Welterweight Championship with a swift 42-second submission victory over Jed Hue. He successfully defended his Polaris title in 2024, securing a decision win against Levi Jones-Leary. Recognizing his exceptional talent and marketability, Polaris signed Szczecinski to an exclusive European contract, solidifying his status as one of the promotion's premier athletes.

Representing Poland at the prestigious ADCC 2022, Szczecinski narrowly lost to Dante Leon in overtime, demonstrating his resilience and competitive spirit. Returning stronger at ADCC 2024, he delivered a standout performance by submitting renowned grappler Garry Tonon via armbar in the opening round of the under-77kg division. Szczecinski advanced to the quarter-finals, ultimately losing on points to PJ Barch. Undeterred, Szczecinski avenged this loss by submitting Barch during a 2025 Polaris Squads team event, further cementing his reputation as a formidable competitor on the international grappling stage.