Existing interventions to address gaming disorder or excessive gaming primarily focus on individuals with the disorder. Little research has been undertaken regarding the development and evaluation of interventions tailored for concerned significant others (CSOs), who often feel distressed due to their loved ones’ excessive gaming. This pilot study adopts a mixed-method design to evaluate the impacts of four sessions of a mentalization-informed motivational interviewing group intervention for CSOs. For the quantitative evaluation, a non-randomized controlled design is used. The study involves 38 parents of adolescents who are experiencing excessive gaming in Hong Kong. The intervention group (n = 23) received the CSO intervention; the control group (n = 15) did not. A total of 12 qualitative interviews with the intervention group participants were also conducted to collect their views on the usefulness of the intervention. The quantitative findings show that the intervention group experienced significant improvements in mental health, emotion regulation, and gaming-related parent–child conflict from pre-test to post-test, compared with the control group. The qualitative data corroborates the quantitative findings and provides insights into potential reasons for these improvements. The findings contribute to the knowledge base regarding CSO interventions in the context of gaming disorder—an underexplored area of study.