The aim of this study was to assess the impact of omega-3 fatty acid supplementation among other nonpharmacological treatments on mental health and quality of life (QOL) of children with behavioral disorders. An observational multicenter study of 6- to 12-year-old children with behavior-related problems was performed in Spain with a three-month follow-up assessment. The Kidscreen-10 and Strengths and Difficulties Questionnaires (SDQ) were used to assess effectiveness of each intervention. Characteristics of study population were compared with those of the general population. Subanalyses of two homogenous subgroups, who received versus did not receive dietary supplementation with omega-3 fatty acids, were performed. The study included 942 children (69.1% male) with a mean (SD) age of 8.5 (1.8) years. Overall, patients' health status and QOL significantly improved at three months (p < .001). Scores on the SDQ also improved, with significant reductions on all subscales (p < .05). Comparison of SDQ results with the same-age general population showed higher overall scores in the study population (8.5 [5.5] vs. 18.6 [8.1], respectively) and on all the subscales (p < .001 in all cases). The omega-3 fatty acid supplementation subgroup presented greater improvements in each category of SDQ (p < .05), except for the emotion subscale. Omega-3 fatty acid supplementation alone or in combination with other nonpharmacological treatments is effective in improving children's mental health. Overall, nonpharmacological recommendations currently made by pediatricians seem to be effective in improving the perceived health status and patients' QOL and in the reduction of health problems, especially hyperactivity/inattention and conduct problems.