Henrik Castrén was a member of a family of lawyers. His father, Väinö Sakari Castrén, had made a career as a lawyer in Helsinki and his grandfather, Johannes Castrén, had served as a juror at the District court of Kuopio.

Henrik Castrén graduated from the university of Helsinki in 1935 and trained on the bench later on the same year. He started his career at his uncle’s law office and continued working for Heikki Borenius Sr. In 1944, Heikki Borenius Sr. offered partnership to Henrik Castrén. With Henrik Castrén as a younger partner in the firm, the majority of the new cases of the firm focused on corporate and contract law.

In 1948, the firm was registered in the Trade Register under the name Attorneys at law Borenius & Castrén.

In 1957, a year prior to his accidental death, Henrik Castrén decided to leave the firm and Heikki Borenius Jr. took over his partnership.