Importance Neurodevelopmental conditions (NDCs) are common, lifelong conditions that typically manifest during childhood. Cumulatively, approximately 15% of children globally have an NDC. NDCs have been associated with increased mortality risk, although extant literature pertains mostly to adults or to specific NDCs. Objective To assess the mortality risk for youth aged 0 to 24 years with and without NDCs, for specific NDC groups, and by sex. Design, Setting, and Participants A national 15-year birth cohort study of 888 975 youth from 1995 to 2009 in Aotearoa New Zealand. Cox proportional hazard models compared mortality risk after adjusting for sociodemographic characteristics. Statistical analysis was conducted from August 17, 2024, to August 27, 2025. Exposures Youth with NDCs. Main Outcomes All-cause mortality was the primary outcome, with medical, injury, and suicide cause-specific mortality as secondary outcomes. Results Of the 888 975 youths in the birth cohort, NDCs were identified in 40 071 (4.5%) (mean [SD] age at NDC diagnosis of 8.5 [4.6] years; 10 605 [26.5%] were female). Mortality risk was significantly higher for youth with NDCs compared with those without NDCs (adjusted hazard ratio [aHR], 4.67 [95% CI, 4.13-5.28]) and higher for female individuals with NDCs compared with female individuals without NDCs (aHR, 9.69 [95% CI, 7.93-11.84]). Mortality risk was higher for all specific NDCs, including specific learning and motor disorders (aHR, 8.50 [95% CI, 7.11-10.16]), intellectual disability (aHR, 8.27 [95% CI, 6.92-9.89]), communication and language disorders (aHR, 6.06 [95% CI, 4.14-8.86]), autism (aHR, 2.58 [95% CI, 1.94-3.42]), and attention-deficit/hyperactivity disorder (aHR, 2.06 [95% CI, 1.67-2.54]) compared with those without these respective conditions. Cause-specific mortality risk for those with any NDC was notably higher for medical-related deaths (aHR, 12.13 [95% CI, 10.31-14.28]) but also significantly higher for injury (aHR, 1.88 [95% CI, 1.42-2.48]) and suicide (aHR, 1.69 [95% CI, 1.21-2.37]) compared with those without NDCs. Conclusions and Relevance This population-level birth cohort study revealed a significantly higher mortality risk among youth with NDCs, especially female individuals, and marked variation in mortality risk by NDC diagnosis. These findings underscore the urgent need for targeted health care interventions, improved access to medical and mental health services, and systemic changes to address the social and structural barriers that contribute to health inequities.