Mental hygiene experienced significant growth on an international level in the first half of the 20th century. A concept of American origin, mental hygiene developed into various forms in different cultural and national contexts. With a large international settlement and vibrant cultural activities, Shanghai witnessed a rise of interest in preventing mental illnesses and promoting mental health during the 1930s and early 1940s. The city gradually became one of the most important places for providing mental hygiene services in China. Apart from the establishment of mental hospitals, departments of neuropsychiatry, and child guidance clinics, people from various disciplines, sectors, and nationalities united to deliver health services to the foreign as well as local Chinese population. The present study first examines the social and cultural conditions that made possible, according to contemporary firsthand accounts, this international “teamwork.” Taking the establishment of The Mercy Hospital for Nervous Diseases and the organization of child guidance clinics as examples, this study investigates the ways in which knowledge and practices of different origins were combined and transformed. In contrast to previous depictions of the development of psychiatry and mental hygiene in Republican China as a product of missionary influence, scientific progress, or social control, this study seeks to illuminate the interplay of international and local forces in negotiating the meaning of mental hygiene and creating a flexible public health model characteristic of Shanghai’s political and social makeup.