The COVID-19 pandemic has posed significant challenges to mental health worldwide, raising the need to identify stable psychological resources that promote sustainable well-being. This longitudinal study examined whether character strengths predict well-being, post-traumatic growth, and mental health over a 13-month period. Participants (N = 146) completed online measures of character strengths, mental health, life satisfaction, affect, and post-traumatic growth at two time points. First, we tested whether a single general factor of character predicted later mental health and whether life satisfaction, affect, and post-traumatic growth mediated this relationship. Then, we repeated this model but with five different character strengths factors as predictors. Results showed that character predicted all mediators and mental health over time, but only the affective components of well-being mediated the relationship between character and mental health, especially positive affect. When looking at the five character strengths factors, although the majority predicted higher well-being and better mental health over time, goodness and interpersonal and fortitude strengths yielded the strongest effects. These findings suggest that character strengths contribute to sustainable well-being by fostering affective resilience in the face of adversity, aligning with the goals of Sustainable Development Goal 3 (Good Health and Well-being).