ALG1-congenital disorder of glycosylation (ALG1-CDG) is an autosomal recessive multisystem disease. We here present a patient with a mild phenotype of ALG1-CDG. A 15-month-old female was referred with hypotonia, failure to thrive, and developmental delay. At 8 months of age, failure to thrive, feeding difficulties and developmental delay became apparent, and an epileptic seizure was observed at 11 months of age. Progressive deterioration and swallowing difficulty were observed. A brain MRI revealed a widening of the cerebrospinal fluid spaces and ventricular system, and decreased protein C, protein S and antithrombin III levels were identified. The isoelectric focusing showed a type 1 pattern. A homozygous c.1076C>T (p.Ser359Leu) variant was found in the gene. CDG should be taken into consideration in patients presenting with unexplained multisystem involvement.