This paper investigates how the presence of a large fraction of rule-of-thumb consumers and an informal sector and (henceforth, informality) impact on the effectiveness of monetary policy in developing/emerging economies. We develop a small open economy New-Keynesian model, which is estimated using data on selected Sub-Saharan African countries where the coexistence of these two frictions is widespread - Burundi, Malawi and Rwanda. The results reveal that (i) rule-of-thumb consumption enhances the dominance of demand shocks and makes inflation stabilization a challenge; (ii) the presence of an informal sector causes supply shocks to be dominant, creating a trade-off between stabilizing inflation and output; (iii) rule-of-thumb consumption weakens the transmission mechanism of monetary policy while its interaction with informality worsens the situation in most of the selected countries; (iv) informality amid a large population of rule-of-thumb consumers causes the nominal interest rate to counterintuitively decline in response to a contractionary monetary policy shock; (v) in some of the selected countries, a positive productivity shock counterintuitively triggers a nominal exchange rate appreciation when informality interacts with rule-of-thumb consumption behavior; (vi) the coexistence of informality and rule-of-thumb consumption behavior amplifies country-risk premium shocks and; (vii) rule-of-thumb consumption behavior is welfare enhancing. These findings are informative to policymakers, particularly in emerging economies, on the priority reforms as they transition to inflation targeting frameworks. Direct policy implications emerge regarding financial inclusion, the size of the informal sector, and farm input subsidy programs.