Gay and bisexual men (GBM) with prostate cancer experience worse sexual and mental health outcomes following prostate cancer treatment than heterosexual men. Emerging evidence suggests that GBM may change their role-in-sex in response to treatment effects. The purpose of this study was to describe the impact of prostate cancer treatment on role-in-sex, to estimate the prevalence of such changes, and to determine the impact on quality of life and mental health. We conducted semi-structured interviews with 30 sexual minority prostate cancer patients. Then, we recruited 401 gay and bisexual prostate cancer patients into a study assessing the effects of rehabilitation. Qualitative data were analyzed using descriptive thematic analysis. Differences in quality of life and mental health outcomes were analyzed using multivariate analyses of variance. Prostate cancer treatment resulted in loss of role-in-sex for many patients. When changes in role-in-sex occurred, the shifts were predominantly from tops to bottoms. Those with a current top role-in-sex had significantly better sexual and mental health outcomes than either versatiles or bottoms. Clinical implications include the need for providers to ask about role-in-sex in order to address disparities in health outcomes by sexual orientation and to provide culturally appropriate care to sexual minority patients.