Background: People affected by mental illness often come from families with patterns of mental illness that span across generations. Hence, child and adolescent mental health services (CAMHS) likely provide treatment to many children with parents who also experience mental illness. Objectives and Methods: The aim of this scoping review was to: (1) identify the prevalence of mental illness among parents of children in CAMHS; (2) identify and appraise the methodologies that have been implemented to assess the prevalence of parental mental illness in CAMHS; (3) identify additional circumstances associated with families where both parent and child experience mental illness; and (4) present recommendations that have been made for CAMHS practice based on these findings. English language, peer-reviewed studies (2010–2018) that had investigated the mental health of parents in CAMHS were included in the review. Findings: Literature searching yielded 18 studies which were found to have utilised diverse methodologies to assess parental mental health. Overall, reported prevalence of parental mental illness ranged from 16 to 79%; however, a single study that was deemed to be comprehensive reported prevalence rates of 36% for mothers and 33% for fathers. Across studies, parent and child mental illness was found to be associated with additional adversities impacting family functioning and wellbeing. Conclusions: For children who receive treatment for mental illness, having a parent who also experiences mental illness is a frequent family circumstance that has implications for their prospects for recovery. Accordingly, the mental health of parents should be an important consideration within the mental health care CAMHS provide to children.