Background: Not much is known about low mood and its associates in child psychiatric patients. In this study, we examined the prevalence of low mood, how it associates with disruptive behaviour, and affects clinician-rated global functioning in child psychiatric outpatients. Methods: The study population consisted of 862 5-12 year-old child psychiatric patients. The study sample was a subsample of all 1251 patients attending a child psychiatric outpatient clinic at Helsinki University Hospital in 2013-2015 formed by excluding 4 year-old and 13 year-old patients and those with missing or incomplete data. The parent-rated Strengths and Difficulties Questionnaire, collected as part of the routine clinical baseline measure, was used as a measure of psychiatric symptoms. The diagnoses were set according to ICD-10 by the clinician in charge after an initial evaluation period. The Children’s Global Assessment Scale (CGAS) score set by clinicians provided the measure of the patients’ global functioning. All information for the study was collected from hospital registers. Associations between emotional symptoms and conduct problems/hyperactivity scores were examined using ordinal regression in univariate and multivariate models, controlling for age and sex. The independent samples T test was used to compare the CGAS values of patient groups with low/normal mood. Results: In our sample, 512 children (59.4%) showed low mood. In multivariate ordinal regression analysis, low mood associated with conduct problems (OR 1.93, 95% CI 1.39-2.67), but no association was found between low mood and hyperactivity. Low mood was prevalent among children with oppositional defiant disorder or conduct disorder (51.8%). The global functioning score CGAS was lower among children with parent-reported low mood (52.21) than among children with normal mood (54.62, p < 0.001). The same was true in the subgroup of patients with no depression diagnosis (54.85 vs. 52.82, p = 0.001). Conclusions: Low mood is prevalent in child psychiatric outpatients regardless of depression diagnosis and it has a negative effect on global functioning. Low mood and behavioural problems are often associated. It is important to pay attention to low mood in all child psychiatric patients. We recommend prevention measures and low-threshold services for children with low mood.