Background <p>Poor mental well-being can affect the future of young individuals, especially during adolescence. This study aimed to investigate the association between parental income and poor mental well-being in youth aged 15–19, while also exploring whether this relationship differs by gender and follows a specific pattern.</p> Methods <p>We conducted a cross-sectional study using data from The Danish National Youth Study, which included 53,038 high school students aged 15–19. This was combined with information from several national registers. Mental well-being was measured using The Short Warwick-Edinburgh Mental Health Well-being Scale, and poor mental well-being was defined as scoring in the lowest decile. Parental income was categorised into quintiles. We used multivariable logistic regression to analyze the data and restricted cubic spline regression to examine the relationship’s shape.</p> Results <p>Compared to the highest parental income group, the adjusted odds ratios of poor mental well-being were 1.04 (95% confidence intervals: 0.94–1.15) for Q4, 1.12 (1.01–1.24) for Q3, 1.20 (1.08–1.33) for Q2, and 1.21 (1.08–1.36) for Q1, with no significant differences between genders. In continuous analysis, we found that higher parental income was associated with less poor mental well-being up to the approximately 75th percentile of income; from this level, higher income was not associated with further change in poor mental well-being.</p> Conclusion <p>Lower parental income was associated with higher odds of poor mental well-being during adolescence, with no gender differences. The findings suggest an indication of a threshold where lower income becomes more strongly associated with higher odds of poor mental well-being.</p>