Hong Kong parents’ psychological control is evident as relatively high, which might have a negative impact on their children’s life orientation and attitude. Optimism plays a central role in young people’s development, psychological well-being, and health. This study aimed to examine 1) the relationship between parental psychological control and optimism; 2) the mediating role of self-mastery between these two concepts. A cross-sectional study was employed to collect/measure Hong Kong middle-school students’ socio-demographic information, parent psychological control (measured by The Chinese Paternal Psychological Control Scale), self-mastery (measured by The Mastery Scale), and optimism (measured by The Chinese version of the 6-item Revised-Life Orientation Test). Multivariate regression analysis and Baron and Kenny’s four-step approach were adopted for the relationships/mediation analyses. A total of 9244 middle school adolescents were included in the data analysis. The average age of the adolescents was 15.11 (SD, 1.79) years (ranges from 11 to 18). The regression analysis showed that both paternal psychological control (β = -0.036, p < 0.001) and maternal psychological control (β = -0.034, p < 0.001) had a significant direct association with optimism. Self-mastery fully mediates the relationship between paternal/maternal psychological control (β = -0.014 or -0.013, p < 0.05) and optimism. Findings support the significant relationships between parental psychological control and adolescents’ optimism levels and the significant mediation role of self-mastery. The findings indicate that psychosocial interventions designed to enhance young people’s self-mastery ability may buffer the negative impact from parental psychological control and improve optimism.