At the onset of the novel coronavirus (COVID-19) pandemic, countries reported elevated rates of psychiatric symptoms. Previous research indicates that subclinical narcissism may reduce depression and stress through mental toughness. The researchers collected data from the United Kingdom (UK) and Greece (GR) on self-reported depression, anxiety, stress, COVID-19 related worry, subclinical narcissism, and mental toughness. Two samples, one cross-sectional (N = 1846) and one semi-longitudinal (N = 184), were used to compare rates of psychiatric symptoms pre and during COVID-19 across the UK and GR, and to test a path model in which subclinical narcissism reduced psychiatric symptoms through mental toughness. From pre to during COVID-19, UK participants exhibited increased depression, lower anxiety, and no change in stress, whereas GR participants showed a decrease in anxiety and stress and consistently low symptoms of depression. Subclinical narcissism exerted a negative indirect effect on psychiatric symptoms through mental toughness in both samples, but a negative total effect on anxiety and stress only in the UK sample. Findings indicate that exploring links between narcissism and prosocial traits can provide novel insights into differences in the adaptive use of personality traits in relation to mental health.