Introduction: People with intellectual disability (ID) face barriers to accessing mental healthcare. Individuals who have had significant experience in intellectual disability mental health (IDMH) can provide insights into strategies to overcome these barriers. Methods: This historical investigation applied an interpretive descriptive approach using semi-structured interviews with fourteen IDMH experts. Thematic analysis was undertaken to explore the delivery of mental health services to people with ID since the 1960s. These themes were then analyzed to identify key lessons about delivering quality mental healthcare. Results: Key lessons suggest the need for: specialized IDMH services, clear policy and governance, including people with ID in the design and delivery of services, holistic and collaborative services, an equipped workforce, quality assessment and treatment, and ongoing research. Conclusions: A historical review of mental health services for people with the ID provided valuable insights into strategies that could be implemented to enhance mental healthcare for this important group.