Background Transitions from Child and Adolescent Mental Health Services (CAMHS) to adult services (AMHS) are often complex and negative experiences for young people and those supporting them. Young people involved with social services are at greater risk of mental ill-health but little is known about their mental health service transitions. Methods This retrospective population-based cohort study used Secure Anonymised Information Linkage (SAIL) data to examine whether young people in Wales with CAMHS contact transitioned to AMHS. Social care status was the primary exposure. Gender, ethnicity, deprivation, health board, and mental health related diagnosis codes in health records were included as covariates. Stratified logistic regression compared odds of CAMHS with odds of transition, and estimated differences in transition by social services involvement. Results Results showed young people receiving care and support (YPRCS) and young people looked after (YPLA) had higher prevalence of diagnosis codes in health records than those with no social services involvement. YPRCS and YPLA were more than twice and nearly four times as likely to access CAMHS (OR = 2.03, 95% CI [1.86, 2.21], p< .001; OR = 3.74, 95% CI [3.07, 4.56], p < .001). However, likelihood of transition was much smaller for YPRCS (OR = 1.18, 95% CI [1.03, 1.35], p = 0.020) and not statistically significantly different for YPLA (OR = 1.27, 95% CI [0.96, 1.98], p = 0.095) when compared with their peers with no social services involvement. Recorded diagnosis codes were more strongly associated with CAMHS access than transition to AMHS. Associations between diagnosis codes and CAMHS were weaker among those involved with social services. Neurodevelopmental condition diagnosis codes influenced transition differently according to social services involvement. Conclusions Despite higher levels of diagnosis codes in health records and greater CAMHS access, young people involved with social services were only marginally or no more likely to transition to AMHS. This suggests potential discontinuity in care. The findings highlight the importance of considering how social services involvement interacts with diagnostic profiles in shaping transition outcomes. Further research is needed to explore mechanisms underlying these differences and to inform targeted support strategies.