During the Covid-19 pandemic, people were instructed to adhere to three key ‘transmission-reducing behaviours’ (TRBs); hand washing, wearing of face coverings, and physical distancing. Consistent enactment of these TRBs requires effortful cognitive functioning but multiple features of the pandemic (stress, social isolation, uncertainty) would be expected to detrimentally affect cognitive functioning. The present study assessed cognitive function in a nationally-representative sample of the general public during the pandemic and investigated its association with psychological distress and role in determining adherence to TRBs. Cognitive functioning was not impaired relative to population norms, but was significantly poorer in females than males and in those reporting higher anxiety and depression. General cognitive functioning did not predict adherence to TRBs, but TRB-specific cognitive functioning (forgetting and planning of specific TRBs) partially mediated the relationship between intentions and adherence for physical distancing and hand washing, suggesting that brief planning interventions and regular reminders may help to boost adherence to these TRBs.