This article discusses the continuity between cosmopolitan collaboration and wartime collaborationism from 1932–1941 by exploring the Chinese Maritime Customs Service (CMCS) and its international staff. The CMCS managed China’s international trade and directed the custom houses in northern China before 1937, and in occupied China and free China from 1937–1941. The customs revenues generated by this international trade were pledged to service China’s international obligations. This article argues that both Chinese and Japanese staff members’ activities to maintain the status quo could be considered as wartime collaborationism from the perspectives of Japan, Manchukuo, and the Collaborationist and Chongqing governments, but all parties tolerated their activities until the outbreak of the Pacific War. The reason for this was that all parties benefitted from the CMCS’s management of international trade and its implementation of international obligations which had existed since the mid-nineteenth century. This article situates wartime collaborationism within the long-existing institutional network that was welcomed as cosmopolitan collaboration in the prewar, wartime, and postwar periods, rather than treating it as a unique wartime setup and ideology. Such a view also illuminates the postwar exchange of personnel and cooperation among former enemies, which grew out of prewar collaboration and wartime collaborationism.