This study investigated the effects of filial therapy on children with externalizing behaviors and mothers’ parenting stress. The results obtained using the Achenbach System of Empirically Based Assessment show that six parent-child pairs among the members had an externalized scale t score above the marginal clinical range (t ≥ 60). Based on the study design, the members were invited to participate in a filial therapy group for 2 hours per week over the course of 10 weeks. The study was also conducted over the course of 10 weeks. The study adapted both quantitative and qualitative research methods; the quantitative method uses a single-subject design. The members were administered the pre- and pro-tests of the Achenbach System of Empirically Based Assessment and the Parenting Stress Index, as well as the Teacher’s Report. After the test, the Wilcoxon signed-rank test was used to determine the results. For the qualitative method, to investigate the experiences of mothers who had children with externalizing behaviors in the filial therapy session, the members were interviewed individually to explore the experiences of mothers for 1 hour after the session was completed. This study provides the following research findings: (1) Filial therapy had significant effects in terms of not only reducing the children’s problems related to internalizing, externalizing, and overall parts but also in rapidly reducing the mothers’ parental stress continuously. (2) The qualitative data analyses showed that the effects of the filial therapy program on the mothers of children with externalizing behaviors include awareness and understanding, positive changes, finding wonderful things in life, and improving both family relationships and communication. The study results show that filial therapy had positive effects on all the members, including by reducing children’s externalizing behaviors and reducing mothers’ parental stress. This study not only proposes relevant recommendations based on the research findings but also provides viable suggestions for future research.