Anxiety disorders in children and adolescents are prevalent and, if left untreated, can lead to comorbid psychological disorders, substance use, poor socioemotional functioning, and academic and occupational underachievement. Subthreshold presentations of anxiety in youth could become problematic if overlooked, resulting in the aforementioned negative outcomes. Thus, it is important to treat such presentations, including with evidence-based treatments such as cognitive–behavioral therapy (CBT). However, because of not meeting the diagnostic criteria for which many CBT protocols were developed, subthreshold presentations of emotional problems could be an especially good match for transdiagnostic treatments like the Unified Protocol for Children (UP-C). Few studies have addressed this application of the UP-C to subthreshold emotion disorders; thus, this study aimed to preliminarily examine in a systemic case analysis (a) the efficacy of the UP-C for a preadolescent patient with mild anxiety/depression, and (b) the clinical utility of assessing both nomothetic and idiographic outcomes. Pre–post and time series outcome data demonstrated significant decreases in symptoms of depression, irritability, and negative reactions to events and significant increases in emotion identification and ownership of emotions. A network analysis of time series data described dynamics between parent and child ratings as well as the importance of child-rated anxiety and parent ratings of child’s ownership of emotions. This case study provides evidence for the efficacy of the UP-C with a preadolescent who displayed subclinical symptoms of an emotional disorder. Furthermore, this case study demonstrates the usefulness of nomothetic and idiographic assessments for treating psychological disorders in youth. (PsycInfo Database Record (c) 2021 APA, all rights reserved)