Purpose This systematic review analyzed the effectiveness of rehabilitation interventions on the employment and functioning of people with intellectual disabilities (ID), as well as barriers and facilitators of employment. Methods This was a systematic review of quantitative, qualitative, and mixed methods studies. The outcomes were employment, transition to the open labor market and functioning. The review included qualitative studies of employment barriers and facilitators. The population comprised people with ID aged 16–68 years. Peer-reviewed articles published in English between January 1990 and February 2019 were obtained from the databases Cinahl, the Cochrane Library, Embase, Eric, Medic, Medline, OTseeker, Pedro, PsycInfo, PubMed, Socindex, and the Web of Science. We also searched Google Scholar and Base. The modified selection instrument (PIOS: participants, intervention, outcome, and study design) used in the selection of the articles depended on the selection criteria. Results Ten quantitative (one randomized controlled, one concurrently controlled, and eight cohort studies), six qualitative studies, one multimethod study, and 21 case studies met the inclusion criteria. The quantitative studies showed that secondary education increases employment among people with ID when it includes work experience and personal support services. Supported employment also increased employment in the open labor market, which sheltered work did not. The barriers to employment were the use of sheltered work, discrimination in vocational experience, the use of class teaching, and deficient work experience while still at school. The facilitators of employment were one’s own activity, the support of one’s family, job coaching, a well-designed work environment, appreciation of one’s work, support form one’s employer and work organization, knowledge and experience of employment during secondary education, and for entrepreneurs, the use of a support person. Conclusions The employment of people with ID can be improved through secondary education including proper teaching methods and personal support services, the use of supported work, workplace accommodations and support from one’s family and employer. These results can be utilized in the development of rehabilitation, education, and the employment of people with ID, to allow them the opportunity to work in the open labor market and participate in society.