We aim to expand existing evidence regarding relationships between sexual orientation and mental health among adults incarcerated in U.S. prisons, especially the differences within the LGB + population (i.e., lesbian or gay, bisexual, another orientation). We used Poisson regression models with robust variance to estimate prevalence ratios for mental health indicators (including illness and care) by sexual orientation using 2016 Survey of Prison Inmates study data (N = 24,848). Of these, 7.4% identified as LGB+. Lesbian or gay, and bisexual adults had a higher prevalence of two or more diagnoses, depressive disorder, anxiety, PTSD and manic disorder compared to straight adults. Identifying with another sexual orientation compared to straight was associated with a higher prevalence of two or more diagnoses, anxiety, PTSD and manic disorder. Similarly, bisexual adults compared to straight had a higher prevalence of prior mental health hospitalization, taking prescription medicines at the time of arrest and since arrest, and using professional mental health help since arrest. Further, lesbian or gay adults, and those identifying with another sexual orientation had a higher prevalence of prior mental health hospitalization, taking prescription medicines and professional mental health help since arrest compared to straight adults. Our results indicate diversity in mental illness and related health care use within the LGB + population and disparities between the LGB + and non-LGB + groups. The need for targeted, supportive programming to improve mental health outcomes during incarceration is identified.