As a public health emergency, a pandemic increases susceptibility to unfavourable psychological outcomes. The aim of the present study was to investigate the buffering role of personal resilience in two aspects of psychological functioning, mental health and stress, among Slovene adults at the beginning of the COVID-19 outbreak. Within five days after Slovenia declared epidemics, 2722 participants (75% female) completed an on-line survey measuring mental health and perceived stress as outcome variables and demographics, health-related variables, and personal resilience as predictor variables. Hierarchical logistic regression analyses demonstrated that women, younger, and less educated participants had higher odds for less favourable psychological functioning during the COVID-19 outbreak. In addition, poorer health indicators and COVID-19 infection concerns predicted diminished psychological functioning. The crucial factor promoting good psychological functioning during the COVID-19 pandemics was resilience, additionally buffering against detrimental effects of demographic and health-related variables on mental health and perceived stress. While previous research suggests that mental health problems increase during pandemics, one way to prevent these problems and bolster psychological functioning is to build individuals' resilience. The interventions should be targeted particularly at younger adults, women, less educated people, and individuals who subjectively perceive their health to be rather poor.