It is starting to be recognised that young people with mental health and/or intellectual disabilities making the transition to adulthood from out-of-home care require focused attention to understand their needs and service requirements. Within the UK jurisdiction of Northern Ireland (NI), young people with mental health and/or intellectual disabilities are over-represented in the population of care leavers and yet very little is known about their specific needs. The overall aim of the study reported here was to examine the profile of care leavers with mental health and/or intellectual disabilities in order to better inform how best to configure child and adult service systems to meet their transitional needs. The research involved a survey completed by social workers working in all of the relevant teams across NI to profile the population. A total of 314 completed surveys were returned (134 in the disability category and 180 in the mental health category), comprising 23% of the total population of care leavers. The study found that almost half (47%) of those with an identified mental illness were not receiving child or adult mental health services and, despite high levels of co-existing mental health and intellectual disability, there were very low levels of engagement with mental health services reported for those with an intellectual disability. More than one in five (21%) of the study sample had attempted suicide in the preceding 12 months. The survey found high levels of school and placement disruption, particularly for young people with mental health needs. Almost half (46%) had experienced three or more placement moves. Whilst almost three quarters of those with mental health needs accessed assistance with employment, 42% of those with an intellectual disability did not. The survey highlights the importance of improving routine data collection to identify the complexity of needs, and for improved access to, engagement with and coordination of services to respond to those needs.