This study builds on existing literature on autism spectrum disorder (autism) and sexual and gender minority (SGM) identities and is the first to examine self-declared frequency rates, associated functional impairments, and treatment engagement levels for sexual minority (SM) and non-SM populations with and without autism cross-sectionally via a large nationwide college-student sample across 3 years. Using data from the American College Health Association-National College Health Assessment (ACHA-NCHA) III, we analyzed data from 81,286 college students (ages 18-25 years old) randomly selected from 75 U.S. colleges and universities. Included ACHA-NCHA-III measures consisted of self-reported demographics, SM and gender minority (GM) identities, autism diagnosis, stress, academic impairment, mental health symptoms, and treatment engagement. We found a 2.8% self-declared frequency of autism in the SM group, compared with 0.9% self-declared frequency among the non-SM sample. Autistic college students with both SM and GM identities had the greatest levels of reported adverse stress and the most negative academic and mental health outcomes (i.e., suicidality), followed by college students with two minority identities (i.e., Autism and SM, SM and GM), across all groups. These findings emphasize the need for mental healthcare providers to consider SM and GM identities in non-autistic and, especially, in autistic college students to assess suicide risk and inform treatment.