Although studies pay increasing attention to how organizational citizenship behavior (OCB) affects work–family conflict, most research ignores the boundary conditions and underlying mechanisms of this relationship. Drawing on goal interdependence theory and conservation of resources theory, this research sees two types of goal interdependence as important boundary conditions of how helping behavior affects work–family conflict. We use a combination of quantitative and qualitative methods to test our theoretical model. Specifically, using two-wave survey data collected from 386 employees and 90 supervisors in a manufacturing company, our quantitative study shows that the interaction of helping behavior with cooperative goal interdependence is positively associated with work–goal progress, whereas its interaction with competitive goal interdependence is negatively associated with work–goal progress. In turn, work–goal progress is negatively associated with work–family conflict. The results further reveal that the indirect effect of helping behavior on work–family conflict via work–goal progress is positive and significant only when the level of competitive (cooperative) goal interdependence is high (low). We use 196 employees from the same organization to conduct our qualitative study, the results of which further substantiate and extend the findings from our quantitative study. Finally, we discuss the theoretical and practical implications of our findings.