Westermark made his debut in Stockholm 1857. He was active at travelling Swedish theater companies, and engaged at the Swedish Theatre in Helsinki in Finland in 1869–70. In 1870, he founded the first travelling theater company in Finland which performed in the Finnish language as well as Swedish, when previously theater in Finland had always been conducted in Swedish. When the Finnish National Theatre, the first Finnish language permanent theater, was founded in 1872 under the management of Kaarlo Bergbom and Oskari Vilho, the theater was staffed by the members of the Westermark theater company, notably Aurora Aspegren and August Aspegren.
- Kungl. teatrarne under ett halft sekel 1860-1910/August Felix Westermark
- Paavolainen, Pentti (2012). "Suomalaisen Teatterin synty". Hiidenkivi (in Finnish). Retrieved 28 May 2019.