Abstract Objective Externalizing behaviors affect children’s functioning in different domains. Previous studies revealed the impact of working memory, mainly through its role in goal keeping, goal-related search, and response selection. This study explored the role of expressive language in the relationship between working memory and externalizing behaviors in children. Method The data were collected from 50 children at a private practice agency (male: 76%; Age M = 4.16, SD = 1.13). All participants were diagnosed with at least one neurodevelopmental disorder. Their index score on the Externalizing Problems from Behavior Assessment System for Children, 3rd Edition, index score on the Working Memory from Stanford-Binet Intelligence Scales, 5th Edition, and expressive language subscale score from Vineland Adaptive Behavior Scales, 3rd Edition were collected. A mediation analysis was performed. Results The mediation analysis results showed a significant indirect effect (B = −0.081, 95% CI [−0.172, −0.015]) of working memory on externalizing behaviors through expressive language. It suggested that poorer working memory was associated with less expressive language, which then resulted in more externalizing behaviors. The direct effect between working memory and externalizing behavior was statistically significant (B = −0.331, p < 0.001), indicating that expressive language only partially mediated the relationship between working memory and externalizing behaviors. Conclusions The study provided empirical support for the mediating effect of expresive langaguge on the relationship between working memory and externaizing behaviors in children from a clinical population. It informs that the use of evidence-based interventions on expressive langauge skills might potentially help address externalizing behaviors in children.