Children's exposure to advertising of unhealthy food and nonalcoholic beverages that are high in saturated fats, salt and/or sugar is extensive and increases children's preferences for, and intake of, targeted products. This systematic review examines the differential potential exposure and impact of unhealthy food advertising to children according to socio-economic position (SEP) and/or ethnicity. Nine databases (health, business, marketing) and grey literature were searched in November 2019 using terms relating to ‘food or drink’, ‘advertising’ and ‘socioeconomic position or ethnicity’. Studies published since 2007 were included. Article screening and data extraction were conducted by two independent reviewers. Quality of studies was assessed using the Newcastle-Ottawa quality scale. Of the 25 articles included, 14 focused on exposure to unhealthy food advertising via television, nine via outdoor mediums and two via multiple mediums. Most studies (n = 19) revealed a higher potential exposure or a greater potential impact of unhealthy food advertising among ethnic minority or lower SEP children. Few studies reported no difference (n = 3) or mixed findings (n = 3). Children from minority and socio-economically disadvantaged backgrounds are disproportionately exposed to unhealthy food advertising. Regulations to restrict unhealthy food advertising to children should be implemented to improve children's diets and reduce inequities in dietary intake.