This study aims to examine the moderating role of individual (sex, symptom severity, and comorbid disorders) and familial (parental stress and parental depressive symptoms) factors on the improvement of organization, time management, and planning (OTMP) skills of youths with attention-deficit hyperactivity disorder (ADHD) following participation of families in a multimodal intervention targeting academic organizational skills. ADHD youths (M = 12.04 years, SD = 0.44; first language: 85.7% French, 11.4% English) under optimized psychostimulant treatment and their parents were randomly assigned to either an intervention group (n = 32; 21 boys and 11 girls) or a control group (n = 38; 28 boys and 10 girls). A two-dimensional questionnaire was used to measure improvement of OTMP skills: "Time Management and Planning and Memorization and Material Management (MMM)". Severity of inattention symptoms (p < 0.01) moderates MMM skills' improvement. The intervention was more efficacious in improving MMM skills of youths with more severe inattention symptoms under optimized psychostimulant treatment. No other moderating factors were observed. The intervention was effective in supporting ADHD youths improve their OTMP skills. The intervention appears to buffer the negative impact of inattention symptoms severity on MMM skills improvement.