BACKGROUND: Looked after children (LAC) and care leavers are young people who have been placed under the legal care of local authorities, in many instances due to a history of abuse and/or neglect. These young people have a significantly increased risk of substance use and mental disorder compared to their peers. The aim of the SOLID study is to assess the feasibility and acceptability of a definitive three-arm multi-centre randomised controlled trial (RCT) that compares the effectiveness of two interventions that aim to reduce risky drug and alcohol use and improve mental health among LAC aged 12 to 20 years with usual care. METHODS: All LAC aged 12 to 20 years residing in four local authorities in North East England will be screened by their social worker for risky drug and alcohol use using the CRAFFT (Car, Relax, Alone, Forget, Friends and Trouble) screening tool. Those who score ≥2 will be invited to take part in the trial after further eligibility checks. Informed consent will be taken and baseline data collected. Participants will then be randomised into either (i) Motivational Enhancement Therapy, (ii) Social Behaviour and Network Therapy, or (iii) control-usual care. Follow-up data will be collected 12 months post-baseline. The baseline and follow-up questionnaires will measure self-reported drug and alcohol use, mental health and well-being and health-related quality of life. The follow-up will also collect data on placement stability and self-reported sexual, antisocial and criminal behaviour. Participants will also be asked about the use of health and social services. A detailed process evaluation, using both qualitative and quantitative methods, will be conducted and involve LAC, their carers, social workers and drug and alcohol practitioners. DISCUSSION: Despite having an increased likelihood of risky substance misuse, there is a lack of evidence outlining specific interventions to decrease drug and alcohol use targeting LAC. This feasibility study will provide the information needed to develop a definitive trial. LAC will benefit from the results of this study and the further development of the interventions. TRIAL REGISTRATION: ISRCTN80786829.