Mental health literacy (MHL) is linked to help-seeking behaviours. Although lay people are not always well aware of mental health conditions, few international campaigns and interventions have been developed to raise awareness across cultures. To investigate MHL cross-culturally and to identify factors that are associated with MHL. Using an online survey, 506 participants (103 Greek, 108 UK, 146 USA, 149 other nationality) read and labelled five vignettes of individuals with Autism, ADHD, Schizophrenia, Bipolar disorder and a healthy control. Factors influencing response, confidence and accuracy were compared across all participants, and the effect of various demographics on accurate labelling was compared between countries. Recognition rates were higher than in previous investigations: 75.5% of participants recognised the Healthy vignette, 71.1% ADHD, 61.7% Autism, 56.6% Schizophrenia and 31.6% Bipolar. MHL varied across different countries, with religion, language spoken, and education having the greatest effects. Personal experience of mental illness partially affected MHL. MHL is relatively high for some mental health conditions, but public knowledge of other conditions is still poor. Factors influencing MHL vary across countries.