Deficits in functioning affect people with first-episode psychosis. Deficits in cognitive performance are common in such individuals and appear to be related to functioning. The present study examined the relationship between the domains of cognitive performance and personal and social functioning, as well as evaluating which cognitive domains are the most closely related to personal and social functioning and whether they explain variations once other clinical and sociodemographic aspects are accounted for. Ninety-four people with first-episode psychosis participated in the study; they were assessed with the MATRICS battery. Symptoms were evaluated with the Emsley factors of the positive and negative syndrome scale. Cannabis use, duration of untreated psychosis, suicide risk, perceived stress, antipsychotic doses, and premorbid intelligence quotient was accounted for. Processing speed, attention/vigilance, working memory, visual learning, reasoning and problem solving correlated to personal and social functioning. Processing speed emerged as the strongest predictor of social and personal functioning and underscores the importance of targeting this domain in treatment. Moreover, suicide risk and excited symptoms were also significant variables in functioning. Early intervention, focusing on improvement of processing speed, may be crucial to the improvement of functioning in first-episode psychosis. The relationship of this cognitive domain with functioning in first-episode psychosis should be studied further. •Processing speed emerged as the strongest predictor of social and personal functioning.•None of the other cognitive domains have a significant effect on functioning after controlling for confounding variables.•Early intervention to improve processing speed may be crucial to the improvement of functioning in first-episode psychosis.