Abstract Background A healthy diet is a key determinant of an individual’s health status and is closely related to mental health and quality of life (QoL); however, the exact nature of the relationship is unknown. This study hypothesized that a higher diet quality score is associated with a lower observance of symptoms of depression and anxiety and a higher QoL. Methods This study evaluated 1,295 adults (521 men; 774 women) aged 19–64 years, who participated in the 2014–2015 National Fitness Award Project. Diet quality was measured by the recommended food score (RFS), and mental health and QoL were assessed by the beck depression inventory (BDI), beck anxiety inventory (BAI), and the World Health Organization QoL–Brief (WHOQoL–BREF). Results After adjusting for covariates, the individuals with depression had a significantly lower RFS value compared to those without depression, and the group with a QoL score above the median had a higher RFS value than the group with a QoL score below the median. These trends occurred in both men and women. Subjects in the highest tertile of RFS showed a lower odds of depression (OR = 0.51, 95% CI = 0.32–0.81, p-trend = 0.0043) and a QoL score below the median (OR = 0.40, 95% CI = 0.30–0.54, p-trend