The formation of mass customization competencies is crucial for the increasing number of manufacturing companies in modern times. This study assessed the relationship between mass customization capability and its determinants on sustainable performance. Additionally, it explores the mediating role of mass customization capability and sustainable performance, while also examining the moderating effects of firm size and cross-border eCommerce in these associations. The study used online survey data from 339 manufacturing small-to-medium-sized enterprises in China to test the hypothesized relationships. The collected data were analyzed using partial least square structural equation modeling and necessary condition analysis. The results indicated that flexible manufacturing competencies, modular product architecture, and customer relationship management are significantly and positively connected to mass customization capability. Moreover, the study observed that mass customization capability and competitive pressure have a significant positive influence on the sustainable performance of Chinese manufacturing SMEs. The findings also revealed that firm size and cross-border e-commerce engagement have a negative and positive moderating effect, respectively, between mass customization capability on sustainable performance, which confirms a relatively higher effect of customization capability on sustainable performance among smaller firms and firms engaged in cross-border eCommerce. Fundamentally, these findings can lead to the development of a comprehensive framework to promote mass customization capability, cross-border e-commerce, and sustainable development of manufacturing small-to-medium-sized enterprises China.