Lassen started his career at the local club
Randers HH. In 2016 he won the
Danish 1st Division. He then signed for
Bjerringbro-Silkeborg Håndbold. In the
2017-18 and
2020-21 seasons he finished 2nd in the Danish league with BSH. In the
2022-23 season he was included in the Danish League all-star team. He made his debut on the Danish national team on 28 April 2021, against
Switzerland. On 26 October 2021, it was announced that Lassen had signed a contract with German
HSV Hamburg. From the 2026-27 season he will join
Rhein-Neckar Löwen. ==References==