Having played a single game on loan with the Pirates in the season prior, on July 31, 2014, Southorn signed a one-year AHL contract with the Portland Pirates.[2] In the 2014–15 season, Southorn was limited by injury an appeared in 26 games with the Pirates for 9 points. He was loaned to ECHL affiliate, the Gwinnett Gladiators on two occasions, totalling 9 points in just 8 games.
On August 5, 2015, unable to secure an AHL contract, Southorn returned on a one-year deal for a second stint with the Fort Wayne Komets of the ECHL.[3]
In the following off-season, Southorn left for abroad as a free agent, signing a one-year deal with Russian second division club, Torpedo Ust-Kamenogorsk of the VHL on August 12, 2016.[4] He made his Kontinental Hockey League debut in the following 2017-18 season, with parent affiliate Barys Astana making 23 appearances for four assists.
On July 24, 2018, Southorn left Kazakhstan after two seasons and agreed to an initial try-out contract with German club, the Straubing Tigers of the Deutsche Eishockey Liga (DEL).[5] Following his release from Straubing, Southorn opted to return to North America and the ECHL, securing a one-year contract with the Florida Everblades on September 28, 2018.[6] In the 2018–19 season, Southorn collected 17 points in 21 games before leaving the club to accept a European contract with HC '05 Banská Bystrica of the Slovak Extraliga on December 10, 2018.