Children and young people (CYP) with long-term physical conditions (LTCs) are four times more likely to develop mental health disorders yet many cannot access Children and Young People’s Mental Health Services (CYPMHS) or evidence-based interventions. This study aimed to understand the reasons for this; presence of an LTC neurodevelopmental disorder, or service requirements. 79 CYP mental health practitioners were randomly assigned to read vignettes depicting a hypothetical referral letter for a child with a mental health condition alone ( n = 27), mental health condition and LTC ( n = 25), or mental health condition and neurodevelopmental disorder (Autism Spectrum Disorder-ASD) ( n = 27), answering questions about their likelihood of accepting the referral and proposed treatment plan. There were no significant differences between accessing CYPMHS or being offered first line evidence-based interventions in those with a LTC or ASD compared to those without. However, additional perceived complexity was frequently provided as a reason for rejecting referrals and not offering evidence-based intervention, with clinicians’ predicted success of intervention significantly lower for these CYP. Clinicians were significantly more likely to suggest adapting the intervention in the LTC and the ASD groups to account for additional perceived complexity. The research suggests a need for additional services for CYP with LTCs and those with neurodevelopmental disorders, as well as training/awareness for clinicians.