Background: Mental health, psychological resilience, and happiness are increasingly treated as interconnected rather than independent constructs within positive psychology and public mental health research. However, the direction, mechanisms, and population-specific patterns of this relationship remain dispersed across a heterogeneous literature. Objective: This systematic review synthesises peer-reviewed evidence on the relationships among mental health, resilience, and happiness, following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020) framework. Methods: A structured search of PubMed, PsycINFO, Scopus, Web of Science, and Google Scholar was conducted, supplemented by manual reference-list screening. Studies were eligible if they reported an empirical or reviewed relationship between at least two of the three constructs (mental health, resilience, happiness/subjective well-being) in a peer-reviewed source. Screening, eligibility assessment, and data extraction followed the PRISMA 2020 flow. Thirty-two studies met inclusion criteria and were synthesised narratively. Results: Across populations including adolescents, perinatal women, healthcare workers, teachers, older adults, and athletes, resilience and happiness were consistently and positively correlated, with mental health functioning as both an antecedent and an outcome of this relationship. Protective factors (social support, optimism, emotion regulation, self-compassion) recurred across contexts, while digital and reflective-practice interventions showed small-to-moderate benefits for resilience and happiness outcomes. Conclusions: The evidence supports a reciprocal, mutually reinforcing model in which resilience helps sustain happiness and mental well-being, while good mental health and positive affect, in turn, replenish resilience resources. Methodological heterogeneity, a scarcity of longitudinal designs, and reliance on self-report measures limit causal inference and point to priorities for future research.