The 22- year-old Ghanaian International joins the relegation threatening side after his Start failed to meet his financial demand during contract extension talks.
“We are very happy to bring in an Isaac in front of an important season sprint. There are 12 matches left and we will do our utmost to have an Elite Series game again in 2021 at Consto Arena.
“Isaac brings us quality in central midfield and is a player we think can be both an important player for Mjøndalen, but also good enough for major leagues.
“He is still young and hungry and fits in well with both our short and long perspective,” Mjøndalen Sports Manager, André Nevstad told the club’s website
Twum joined Start in 2018 after impressing in the 2017 Wafu Cup of Nations and helped them to secure promotion into the to-flight league
Twum has played 10 games and one goal for IK Start this season and will hope to help his new team to get a move away from 15th position.