Drawing upon person-environment fit perspective, this study examines the joint and interactive influences of personal competence and environmental characteristics on the happiness of ageing adults around the period of COVID-19 pandemic. Data was collected in two rounds, before and during the pandemic, with 2,028 participants aged 55 years and older in Hong Kong. Personal competence encompassed financial status, physical health, and mental capital, while environmental characteristics included experiences of ageism, perceived social conditions, and age-friendly policies. Ordinary least squares regression was used to examine personal and environmental influences on happiness. Results indicated a significant decline in happiness following the onset of COVID-19 pandemic. Mental capital was found to have the strongest positive influence on happiness, followed by physical health, financial status, and social conditions. Mental capital mitigated the negative relationship between experiences of ageism and happiness. Practical interventions are informed to improve the well-being of ageing adults during pandemic.