Analysis of MafB^−/− mice has suggested that the MAFB transcription factor was essential to islet α- and β-cell formation during development, although the postnatal physiological impact could not be studied here because these mutants died due to problems in neural development. Pancreas-wide mutant mice were generated to compare the postnatal significance of MafB (MafB^Δpanc) and MafA/B (MafAB^Δpanc) with deficiencies associated with the related β-cell-enriched MafA mutant (MafA^Δpanc). Insulin⁺ cell production and β-cell activity were merely delayed in MafB^Δpanc islets until MafA was comprehensively expressed in this cell population. We propose that MafA compensates for the absence of MafB in MafB^Δpanc mice, which is supported by the death of MafAB^Δpanc mice soon after birth from hyperglycemia. However, glucose-induced glucagon secretion was compromised in adult MafB^Δpanc islet α-cells. Based upon these results, we conclude that MafB is only essential to islet α-cell activity and not β-cell. Interestingly, a notable difference between mice and humans is that MAFB is coexpressed with MAFA in adult human islet β-cells. Here, we show that nonhuman primate (NHP) islet α- and β-cells also produce MAFB, implying that MAFB represents a unique signature and likely important regulator of the primate islet β-cell.