This article examines changing governing practices in the context of the Sino-Vietnamese border in the Guangxi Autonomous Region of China. The groups inhabiting the mountainous ranges of this ethnically diverse part of Southeast Asia evaded the reach of the state until the 1970s, when China and Vietnam started tightening control over the land border after the border war. With an increasingly rigid and clearly delimited Sino-Vietnamese borderland, binary forms of classification began to replace earlier fluid identifications, and the room for diverse social and cultural expressions became restricted. It is within this context that the ethnic marriage practices straddling the borders of China and its southern neighboring states discussed in this article took place. Cross-border ethnic Yao marriages have changed from customary to illegal in recent years as a result of Chinaâ€™s strict population control, its changing demography, and the accelerated shortage of manual labor in its border area. Border politics have permeated the private sphere, transforming common ethnic marriage partners into illegal migrants. Although ethnic marriage partners are relegated to an illegal status, they are indispensible in the local labor market and moral economy as mothers, caretakers, translators, guides, and manual workers. This article argues that, despite being antithetical to the bordering logic of state sovereignty, they are important agents who depend on and capitalize on the border economy.