Myelin oligodendrocyte glycoprotein antibody‐associated disease (MOGAD) causes major disability as a consequence of recurrent demyelinating events and neuronal loss. Biomarkers identifying different phenotypes of recurrence or tissue damage might be useful to guide individualized therapy. Herein, we evaluated serum neurofilament light chain (sNfL) as a potential biomarker in both adult and pediatric MOGAD patients. Forty‐nine patients with MOGAD (37 adults, 12 children) and 71 healthy controls (HCs) (56 adults, 15 children) were enrolled prospectively from September 2019 to April 2021 at the Third Affiliated Hospital of Sun Yat‐sen University and the Children's Hospital, Zhejiang University School of Medicine. sNfL levels were determined using ultrasensitive single‐molecule array assay and correlated with clinical parameters. The sNfL levels in MOGAD adults in a relapsed state (median: 31.0 pg/ml) were higher than those in a remission state (8.1 pg/ml, p = 0.001) and in HC adults (10.3 pg/ml, p = 0.004). Similar results were observed in children (relapse: 46.8 pg/ml vs. remission: 13.1 pg/ml, p = 0.001; and vs. HCs: 8.2 pg/ml, p = 0.007) sNfL levels were correlated with recent relapses within 60 days (multivariate: β = 2.02, p = 0.003), seizures (multivariate: β = 2.50, p = 0.021) and brain lesions on magnetic resonance imaging (MRI) of a recent relapse (multivariate: β = 1.72, p = 0.012). Our study showed that sNfL levels are beneficial for identifying recent relapses and seizures and suggest that adult and pediatric MOGAD patients had similar sNfL levels. This study determined the level of serum neurofilament light chain (sNfL) by ultrasensitive single‐molecule array assay. The authors found that the sNfL was significantly higher in relapse than in remission in both adult and pediatric patients with myelin oligodendrocyte glycoprotein antibody‐associated disease (MOGAD). sNfL levels were significantly increased in the brain phenotype in MOGAD patients compared with healthy controls and were beneficial for identifying recent relapses and seizures. These findings are of great importance because sNfL is a promising biomarker for identifying phenotypes of recurrence in MOGAD, which might be useful to guide individualized therapy and reduce burdens on patients.