Background: Social cognitive deficits play an important role in social functioning in adults with established psychosis (schizophrenia), young adults recently diagnosed with schizophrenia in the last 2 years (first episode psychosis; FEP), and teenagers at clinical high risk for developing psychosis (CHR). In particular, the ability to infer other people’s mental states has been shown to be compromised in those with psychosis, albeit less so in earlier stages. Recent studies have shown that processing speed (PS) may be upstream to social cognitive abilities in psychosis. If such were the case, remediating PS may in turn have beneficial effects on social cognition. This study examined whether improving PS would lead to subsequent improvements in social cognition in individuals with schizophrenia, FEP, and/or CHR. We examined whether (a) improving PS was related to improvements in social cognition, and (b) if so, whether these social cognitive improvements would in turn lead to fewer symptoms and better social function. Methods: Participants were 36 adults with schizophrenia (18–55 yo), 18 young adults with FEP (18–26 yo), and 17 teenagers at CHR (12–18 yo), enrolled in a 3-month PS training program. Assessments of PS, social cognition, and symptoms were administered at baseline and post-treatment. Results: Improvement in PS from baseline to post was related to improvement in the ability to determine other people’s mental states in CHR and FEP but not in schizophrenia. This improvement in higher order social cognitive performance was related to fewer negative symptoms and less avoidance of new social situations in the CHR group. No relationship between improvement in PS and symptoms or social function was noted in schizophrenia. Conclusion: Information processing speed deficits may be an important therapeutic target in remediating social cognitive deficits in psychosis, especially at the prodromal stages where there perhaps is more neural plasticity for change and the possibility of mitigating the trajectory of social decline in psychosis.