 Prior to the new Superliga-season, Marcussen signed a new contract with Lyngby on 25 June 2019, keeping him a part of "De Kongeblå" until 2021.Marcussen has represented Denmark at every age group from under-18 to under-21 level.On 22 March 2018, he made his first appearance for the Denmark under-21 team in a 5–0 friendly win over Austria in Wiener Neustadt.