After her husband’s death, Teresia Lönnström served as director of the Lönnström Group for thirty years. From a handful of companies that originally manufactured fuses and artillery shells, the Lönnström Group expanded and diversified after World War II to become one of the largest metal industry enterprises in Rauma. In 1973, as Teresia Lönnström neared her 80th birthday, she sold her majority stake in the group to the Huhtamäki conglomerate. She made several major donations at the time, including to the Finnish Cultural Foundation. She also expanded her collection of Finnish paintings to cover contemporary art.
Teresia Lönnström involved herself in the social activities of the Lönnström Group and played an active role in charities and various societies.