Färöischer Fußballpokal
Thomassen has been the manager of KÍ Klaksvík in the Faroese Premier Division since 2015. During this period he has won the Faroese championship in 2019. The club was runner-up in 2016, 2017. In 2016 he won the Faroe Islands Cup with KÍ. In 2020 KÍ won the Faroe Islands Super Cup for the first time.