Conspiracy beliefs have spread during the Covid-19 pandemic. It is important to understand them because of their potential to undermine trust in societal institutions and willingness to get vaccined. In the present research (N = 538), we assessed the links between conspiracy beliefs, trust in institutions (e.g., government, WHO), and attitudes towards the Covid-19 vaccination across the USA, Brazil and the UK. A moderated mediation analysis revealed the crucial role of political leaders in linking conspiracy beliefs with vaccination attitudes. Trust in the president was positively associated with conspiracy beliefs in Brazil because of its conspiracist president at the time (Bolsonaro), which in turn was negatively associated with vaccination attitudes. In contrast, trust in political leaders at the time in the UK (Johnson) and the USA (Biden) was negatively associated with conspiracy beliefs. In conclusion, our findings contribute to understanding the underlying mechanisms that link conspiracy beliefs with trust and vaccination attitudes.