Children and young people (CYP) with neurodevelopmental disorders (NDDs) may be particularly vulnerable to adverse mental health effects due to the COVID-19 pandemic. We conducted a cross-sectional U.K parent-reported study from 2nd April-2nd June 2020, using the Strengths & Difficulties Questionnaire. CYP with NDDs (n=371) compared to neurotypical controls, had a higher prevalence of emotional symptoms (42% vs 15%), conduct problems (28% vs 9%), and lower prosocial behaviours (54% vs 22%). Those with attention-deficit/hyperactivity disorder showed inflated conduct, and those with autism spectrum disorder exhibited decreased prosocial behaviours. Females with ASD had higher emotional symptoms compared to males.