Background: Individuals with autism spectrum disorder (ASD) and intellectual disability (ID) are at increased risk of potentially traumatic events and may be at increased risk of post-traumatic stress disorder (PTSD). However, knowledge regarding identification of PTSD in this population is limited. The aim of this study was to investigate clinical experience regarding PTSD and trauma assessment in individuals with co-occurring ASD and ID. Method: Interpretative phenomenological analysis was used to explore experiences of identifying PTSD in this population among 18 mental health clinicians working with ASD and ID. Results: Informants viewed PTSD in individuals with ASD and ID as equivalent to PTSD in the general population, but with causes and expressions potentially differing. Several factors were described to contribute to challenges in identification. Conclusions: Trauma may have severe impact in individuals with ASD and ID. Multidimensional, individualized assessment strategies seem necessary to recognize PTSD or trauma-related symptoms in this population.