Purpose: This systematic review was undertaken to determine the effectiveness of motivational interviewing (MI), by social workers, on service user outcomes. Methods: A literature search was undertaken between 2007 and 2018. All eligible studies were analyzed using the Critical Appraisal Skills Programme tool. As heterogeneity was high, a narrative synthesis approach was employed, using thematic analysis for categorizing data. Results: Eleven studies met the inclusion criteria and were included in this review. MI had a positive effect on service user experience, but this was not consistent. Training was variable, but the evidence suggests that practitioner’s need ongoing training, supervision, or coaching while providing MI. Discussion: There is a paucity of research examining the impact of MI on children, which was a limitation of this review. There is a need for more qualitative research to surface views and experience of service users to determine why MI is effective.