PurposeEmotional and behavioral problems in children and young people (CYP) have increased over the pandemic. Those with pre-existing mental disorders are more vulnerable but have been understudied. We investigated emotional and behavioral outcomes in this population; differences across diagnostic groups; and social, educational, and clinical determinants.MethodsWe invited 5386 caregivers and CYP (aged 5–17) under child mental health services pre-pandemic to complete an online survey on CYP’s emotional/behavioral symptoms and pandemic-related circumstances, and integrated responses with clinicodemographic information extracted from electronic health records. We compared four parent-rated outcomes (total emotional/behavioral scores and emotional/behavioral changes as compared to before the pandemic) across the three most common diagnostic groups in our population (Attention Deficit Hyperactivity Disorder (ADHD), Autism Spectrum Disorder (ASD) and emotional disorders (EmD)). We then estimated the association of clinicodemographic and pandemic-related characteristics with emotional/behavioral outcomes.ResultsA total of 1741 parents (32.3%) completed the survey. Parents of CYP with ADHD or ASD reported more behavioral difficulties (t(591) = 5.618 (0.001); t(663) = 6.527 (0.001)); greater emotional deterioration (t(591) = 2.592 (0.009); t(664) = 4.670 (< 0.001); and greater behavioral deterioration (t(594) = 4.529 (< 0.001); t(664) = 5.082 (< 0.001)) as compared to the EmD group. Those with ASD and EmD showed more emotional difficulties than ADHD (t(891) = − 4.431 (< 0.001); t(590) = − 3.254 (0.001)). Across diagnoses, poor parental mental health and challenges with education were most strongly associated with worse outcomes.ConclusionsWithin our clinical population, CYP with ADHD/ASD were the most adversely affected during lockdown. Enhancing clinical service provision that tackles parental stress and supports education may help mitigate the impact of future restrictions.