ADAR1-mediated A-to-I RNA editing is a self-/non-self-discrimination mechanism for cellular double-stranded RNAs. ADAR mutations are one cause of Aicardi–Goutieres Syndrome (AGS), an inherited paediatric encephalopathy, classed as a “Type I interferonopathy.” The most common ADAR1 mutation is a proline 193 alanine (p.P193A) mutation, mapping to the ADAR1p150 isoform-specific Zα domain. Here, we report the development of an independent murine P195A knock-in mouse, homologous to human P193A. The Adar1P195A/P195A mice are largely normal and the mutation is well tolerated. To understand the consequences of the compounded P195A mutations found in AGS, we have generated Adar1P195A/- mice. In this case, the P195A mutation was paired with an Adar null allele similar to that found in patients. Approximately half of the Adar1P195A/- animals are runted with a shortened lifespan, while the remaining are normal. The phenotype of the Adar1P195A/- animals is both associated with the parental genotype and partly non-genetic/environmental. Complementation with an editing-deficient ADAR1 (Adar1P195A/E861A) or the loss of a viral dsRNA sensor MDA5 rescues phenotypes in the Adar1P195A/- mice. These results put MDA5 as the primary initiator of the unwanted immune activation and disease pathologies associated with ADAR1 mutations. Normal survival of Adar1P195A/E861A indicates that the expression of ADAR1 protein, even with impaired RNA editing function is required to prevent pathology development.