Food insecurity is widely recognized as an important social determinant of mental health, yet the processes linking food insecurity to mental well-being remain insufficiently understood. This study conducted two separate longitudinal analyses to examine (1) whether loneliness mediates the association between food insecurity and mental health, consistent with the stress proliferation perspective, and (2) whether loneliness moderates this association, consistent with the stress amplification perspective. Data came from two waves of the UK Household Longitudinal Study (N = 40,288 person-years), including the most recently released wave as of December 2025. Fixed-effects regression models were used to estimate within-individual changes over time. Results indicated that food insecurity was associated with poorer mental health over time. In the mediation analysis, loneliness accounted for part of this association. In the moderation analysis, the adverse association between food insecurity and mental health was stronger among individuals reporting higher levels of loneliness. From a social work perspective, these findings underscore the value of interventions that combine material support with strategies that strengthen social connectedness and emotional support. Integrating food assistance with relationship-building and community-based supports may help reduce the mental health burden of food insecurity and promote more equitable well-being.