Abstract Background Assertive community treatment (ACT) and Flexible assertive community treatment (FACT) are organisation models for intensive assertive outreach that were originally developed for individuals with severe mental illness. The models are increasingly applied to people with mild intellectual disability (MID) or borderline intellectual functioning (BIF) and challenging behaviour or mental illness. Research on these types of care for this population is limited. To gain experience in FACT MID/BIF in the Netherlands and to obtain insight in its outcomes, four organisations specialised in the treatment of individuals with MID/BIF and challenging behaviour participated in a 6‐year implementation and research project. Methods A longitudinal study was set up to investigate outcomes over time. Outcome measures concerned admissions to (mental) health care, social and psychological functioning, (risk of) challenging and criminal behaviour, social participation and client satisfaction. Data were analysed using descriptive statistics and linear mixed models. Results Over time, clients showed improvement in their social and psychiatric functioning and living circumstances. The number of admissions to (mental) health care diminished as well as the number of contacts with police and justice, the level of social disturbance and the risk factors for challenging and criminal behaviour. Problems related to finances, work and substance abuse remained unchanged. Conclusions The results are encouraging and give rise to continued development of and broader research on FACT MID/BIF.