[5] He split the 2013 season between the GCL, the Jamestown Jammers, and the West Virginia Power, hitting a combined .228/.323/.251 with 15 home runs.

Mathisen split the 2018 season between Altoona and the Indianapolis Indians, hitting a combined .261/.363/.440 with 10 home runs and 48 RBI.

[7] He split the 2019 season between the AZL Diamondbacks and the Reno Aces, hitting .288/.412/.592 with 23 home runs and 64 RBI.

[10][11] On September 6, 2020, Mathisen was promoted to the major leagues and he made his debut the next day against the San Francisco Giants.

[15] Mathisen played in 14 games for the Triple-A Tacoma Rainiers, struggling to a .128/.300/.191 line before he was designated for assignment by Seattle on July 16.

[18] On August 29, 2021, Mathisen signed a minor league contract with the San Francisco Giants.