This document outlines lessons learned from over 10 years of mainstreaming internationalization at the University of Helsinki. It discusses how internationalization has become embedded across the university through a decentralized model rather than a centralized international office. Key lessons include that internationalization depends on people not structures, it happens gradually over time, and different parts of the university like faculties and departments require different approaches compared to the central administration. Ongoing leadership commitment is still needed to support internationalization even in a decentralized model.