We assessed mania/hypomania and a range of other symptoms, substance use, and personality dimensions across interview and self-report methods toward advancing understanding of mania/hypomania’s multimethod assessment. Our study, informed by transdiagnostic, dimensional models examined these issues in a clinically-oriented sample (N = 245; mean age = 32, SD =12; 59% currently accessing psychotherapy and/or medication), and as a secondary aim, we evaluated the structure of lifetime interview ratings of hypomania symptoms. We found evidence for a single-factor structure of interview-rated hypomania, and interview-rated hypomania associated strongly with psychosis and related variables, consistent with recent classification-focused research. Self-rated mania/hypomania-relevant measures shared these associations to some degree, but there were also key differences in associations across measures and methods (e.g., self-rated euphoria showed distinctive positive associations with extraversion). We draw on these results to provide ideas for modeling mania/hypomania symptom heterogeneity more thoroughly in future studies and for sharpening multimethod clinical assessment.