Mathisen's main source of inspiration was Fats Waller, both his piano play and his singing, but the later years of his career saw some bebop influence.

He is also remembered for his characteristic appearance with his flat, centrally parted hair, his thin moustache and, most notably, his fondness of large cigars.

In 1936 Mathisen formed his own band and until 1952 he led various acclaimed orchestras which performed in restaurants in Copenhagen and toured in Scandinavia after the end of World War II also in Germany.

Among the members of his band were Erik Parker, Henry Hagemann and John Steffensen, while the trumpeter Jørgen Ryg joined the ensemble in the years after the war.

During the war, a time which has been labelled as the Golden Age of Danish Jazz, he reached huge popularity, particularly at the München establishment where he performed regularly for several years during the Occupation of Denmark.

With Erik Parkerm he also created Danish lyrics to songs such as De 24 Røvere that Fats Waller recorded in 1941 and Jack McVea's "The Key's in the Mailbox".