Abstract Objectives Long-term immunomodulatory therapy of pediatric onset-multiple sclerosis (POMS) is based mainly on published case series and internationally agreed guidelines. Relevant studies in the Greek population are absent from the literature. The purpose of this study is to present data on the efficacy and safety of the 1st line immunomodulatory drugs in the treatment of POMS patients. Materials and methods The present study included 27 patients meeting the IPMSSG criteria for POMS and who are monitored at the outpatient clinic of the Multiple Sclerosis and Demyelinating Diseases Unit (MSDDU), of the 1st Neurological Department, University Hospital of Aeginition. All patients received 1st line immunomodulatory drugs as initial therapy. Clinical, laboratory, and imaging parameters of the disease were recorded before and after treatment. Results Post-treatment, a significant reduction of the relapse number (mean ± SD: 2.0 ± 1.0 vs 1.2 ± 1.6, p = 0.002), EDSS progression (mean ± SD: 1.5 ± 0.8 vs 0.9 ± 0.7, p = 0.005) and ARR (mean ± SD: 1.5 ± 0.7 vs 0.4 ± 0.5, p = 0.0001) was observed, while no changes were observed in the EDSS score, (mean ± SD: 1.8 ± 0.6 vs 1.9. 0.6, p = 0.60). Advanced age at treatment initiation increased the risk for drug discontinuation before 24 months of therapy (HR = 0.6, 95% CI (0.35–0.99), p = 0.04). Conclusions Most pediatric patients are forced to switch to either more efficacious 1st line or 2nd line drugs. Additionally, our study suggests that older age at the time of the 1st line treatment initiation, contributes to earlier drug discontinuation.