Aicardi-Goutières syndrome (AGS) is a genetically determined encephalopathy usually inherited as an autosomal recessive trait. The syndrome can be caused by mutations in the AGS1 gene encoding the exonuclease TREX1, or in any of the AGS2, AGS3, or AGS4 genes that encode the three subunits of the human ribonuclease H2 (RNaseH2) complex. Typically, AGS has an early onset, usually manifesting by the age of 4 months. We describe a female infant in whom the onset of the neurological symptoms of AGS occurred after the age of 12 months, and her younger brother who was identified to be affected by AGS at the age of 8 months on the basis of the presence of non-neurological features alone. This paper is important in providing a detailed description of the late-onset presentation of AGS in patients with proven pathogenic mutations and highlights the occurrence of both chilblains and abnormal neuroimaging many months before the onset of neurological features. Our paper also considers the possible value of immunomodulatory therapy in AGS.