Objective: To assess real-world effectiveness of initial treatment with newer compared to injectable disease-modifying therapies (DMTs) on disease activity in pediatric multiple sclerosis (MS) and clinically isolated syndrome (CIS). Methods: This is a cohort study of children with MS/CIS followed at 12 clinics in the US Network of Pediatric MS Centers, who received initial therapy with newer (fingolimod, dimethyl fumarate, teriflunomide, natalizumab, rituximab, ocrelizumab) or injectable (interferon-β, glatiramer acetate) DMTs. Propensity scores (PSs) were computed, including preidentified confounders. Relapse rate while on initial DMT was modeled with negative binomial regression, adjusted for PS-quintile. Time to new/enlarging T2-hyperintense and gadolinium-enhancing lesions on brain magnetic resonance imaging were modeled with midpoint survival analyses, adjusted for PS-quintile. Results: A total of 741 children began therapy before 18 years, 197 with newer and 544 with injectable DMTs. Those started on newer DMTs were older (15.2 vs injectable 14.4 years, p = 0.001) and less likely to have a monofocal presentation. In PS-quintile–adjusted analysis, those on newer DMTs had a lower relapse rate than those on injectables (rate ratio = 0.45, 95% confidence interval (CI) = 0.29–0.70, p < 0.001; rate difference = 0.27, 95% CI = 0.14–0.40, p = 0.004). One would need to treat with newer rather than injectable DMTs for 3.7 person-years to prevent 1 relapse. Those started on newer DMTs had a lower rate of new/enlarging T2 (hazard ratio [HR] = 0.51, 95% CI = 0.36–0.72, p < 0.001) and gadolinium-enhancing lesions (HR = 0.38, 95% CI = 0.23–0.63, p < 0.001) than those on injectables. Interpretation: Initial treatment of pediatric MS/CIS with newer DMTs led to better disease activity control compared to injectables, supporting greater effectiveness of newer therapies. Long-term safety data for newer DMTs are required. ANN NEUROL 2020 ANN NEUROL 2020;88:42–55.