In 1993, Barnum returned to the Division II level as offensive line coach and run game coordinator at American International College.

After his stint at Coast Guard, Barnum returned to the Division I level at Idaho State and started as offensive line coach in 1998 under Tom Walsh.

[5] Barnum led the Vikings to a 24–17 upset over the Washington State Cougars at Martin Stadium in Pullman, WA on September 5, 2015 for his first career victory as head coach.

[6] On October 10 Portland State defeated North Texas 66–7, giving Barnum and the Vikings their second victory over an FBS school in 2015.

In response to the comments, SUU fans showed up to the game en masse, setting a single-game school attendance record.

[4] In July 2017, Barnum was publicly disciplined by Portland State University following the revelation of an inappropriate relationship with a married female staff member.