Objectives. Periods of social isolation are associated with loneliness in children and young people, and loneliness is associated with poor mental and physical health. Children and young people with pre-existing mental health difficulties may be prone to loneliness. Containment of COVID-19 has necessitated widespread social isolation, with unprecedented school closures and restrictions imposed on social interactions. This rapid review aimed to establish what is known about the relationship between loneliness and mental health problems in children and young people with pre-existing mental health problems. Methods. We sought to identify all primary research that examined the cross-sectional and longitudinal associations between loneliness/perceived social isolation and mental health in children and young people with pre-existing mental health problems. We also aimed to identify effective interventions that reduce the adverse impact of loneliness. A rapid systematic search was conducted using MEDLINE, PsycINFO, and Web of Science. Results. Of 4,531 papers screened, 15 included children and young people with preexisting mental health conditions. These 15 studies included 1,536 children and young people aged between 6 and 23 years with social phobia, anxiety and/or depression, and neurodevelopmental disorders. Loneliness was associated with anxiety and depression both cross-sectionally and prospectively in children and young people with mental health problems and neurodevelopmental conditions. We found preliminary evidence that psychological treatments can help to reduce feelings of loneliness in this population. Conclusions. Loneliness is associated with depression and anxiety in children and young people with pre-existing mental health conditions, and this relationship may be bidirectional. Existing interventions to address loneliness and/or mental health difficulties in other contexts may be applied to this population, although they may need adaptation and testing in younger children and adolescents.