The Arenaviridae is a large family of viruses causing both acute and persistent infections and causing significant public health concerns in afflicted regions. A “trademark” of infection is the quick and efficient immuno-suppression mediated in part by a 3’-5’ RNA exonuclease domain (ExoN) of the Nucleoprotein (NP). Mopeia virus, the eastern African counterpart of Lassa virus, carries such ExoN domain, but does not suppress the host innate immunity. We have recently reported the crystal structure of the Mopeia virus ExoN domain, which presents a conserved fold and active site. In the present study, we show that the ExoN activity rules out a direct link between ExoN activity and alteration of the host innate immunity. We found that the Arenavirus ExoN, however, is able to excise mis-incorporated bases present at the 3’-end of double stranded RNA. ExoN(-) arenaviruses cultured in cells dampened in innate immunity still replicated in spite of a significant reduction in the viral charge over several passages. The remaining ExoN(-) virus population showed an increased base substitution rate on a narrow nucleotide spectrum, linking the ExoN activity to genome editing. Since, the Arenavirus ExoN belongs to the same nuclease family as that of the nsp14 coronavirus ExoN; which has been recently shown to promote viral RNA synthesis proofreading; we propose that Arenavirus ExoN is involved in a “limited RNA editing” mechanism mainly controlled by structural constraints and a low mutational/fitness ratio.