Background: Parents play a key role in the transition process of their children. After high school students with intellectual disabilities leave the school system, they need the cooperation of schools and families to ensure successful employment and independent living. Methods: A case study was conducted in a Taiwanese high school special education class, collecting data from 12 participants through interviews, documents and diaries. Results: The results showed that active parental involvement in transition activities, strong parent-teacher partnerships, understanding of students' strengths and support from social resources were key to successful transitions. Conclusions: This study explored the effectiveness and difficulties of parents' participation in the post-school transition of students with intellectual disabilities from the perspectives of teachers, parents and students. Finally, it proposed the need to be the child's significant other, early participation in transition planning and joint involvement of parents and their children.