Specific externalizing attributional biases appear to be common in early psychosis. They may represent trait risk factors for the later development of a psychotic disorder, yet few studies have investigated this in clinical "at risk" populations. We aimed to investigate one particular bias, the Locus of Control of reinforcement (LOC) in a "Ultra High Risk" (UHR) for psychosis group. We recruited UHR individuals from an established at risk clinical service and a community control group. LOC was measured using the Adult Nowicki Strickland Internal External scale (ANSIE). Neuropsychological functioning, social functioning and psychopathology were assessed. We analyzed data from 30 controls and 30 UHR individuals. The UHR sample had a significantly more externalized LOC (control for events perceived to be external to the person) than controls. This difference remained statistically significant after adjusting for covariates (age, gender and IQ). More externalized LOC scores were negatively correlated with social and occupational functioning scores in the control group but not in the UHR group and positively correlated with negative symptoms and paranoid symptoms in the UHR group. These findings have implications for identifying potential psychological vulnerabilities for the development of psychosis and informing treatment approaches within the at risk group.