Donner Family. In 1926, Lauri Borenius, cousin to Heikki Borenius Sr., starts working at the office handling mainly legal issues of the Donner family. Among other things, Lauri Borenius takes care of issues relating to the family mansion, Ahdenkallio. Later on, Lauri Borenius acts also as managing director of the Donner family company Oy Ahdenkallio-Hyvinkää Ab.

The Finnish Opera. Heikki Borenius Sr. acts as a board member as well as the chairman of the Finnish Opera until 1948 when the Foundation of the Finnish National Opera is founded. He acts also as the foundation’s chairman until the end of the 1950s.

Artists. Heikki Borenius Sr. advises e.g. his brother-in-law, Eero Snellman (1890 - 1951), one of the time’s foremost artists and also the designer of the Finnish flag. Heikki Borenius senior drafts the will of the great Finnish composer, Leevi Madetoja (1887 - 1947) and his wife, renowned poet L. Onerva Madetoja.