Introduction: Psychosis and mania share conceptual, genetic and clinical features, which suggest the possibility that they have common antecedents. Participants identified to be at-risk for psychosis might also be at-risk for mania. We aimed to identify the rate and predictors of transition to mania in a cohort of youth with clinical or familial risk for psychosis. Methods: Among a cohort of 416 young people with an at-risk mental state for psychosis defined using the Ultra-High-Risk (UHR) criteria, 74.7% were followed up between 5 and 13 years from their baseline assessment. We undertook a matched case-control examination of those who developed mania over the follow-up period compared to those who did not develop mania or psychosis. Transition to mania was determined using either a structured clinical interview, or diagnoses from a state-wide public mental health contact registry. Clinical characteristics and risk factors were examined at baseline using information from structured interviews, clinical file notes, rating scales and unstructured assessments. Results: Eighteen participants developed mania (UHR-Manic transition or UHR-M, 4.3%). In comparison with participants matched on age, gender and baseline-study who developed neither mania nor psychosis, more UHR-M participants had subthreshold manic symptoms or were prescribed antidepressants at baseline. They also had lower global functioning. Discussion: In addition to the UHR criteria, features such as subthreshold manic symptoms and antidepressant use may help identify at-risk groups that predict the onset of mania in addition to transition to psychosis. Presence of manic symptoms may also indicate syndrome specificity early in the prodromal phase.