Display omitted •Effect of millimeter-sized geometric features on dropwise condensation was studied.•Heat transfer performance with/without air’s presence was experimentally examined.•The same geometric feature can show opposite effects on condensation heat transfer.•Numerical models were developed from the analysis of the thermal resistance network. In this work, we investigated the effects of millimetric surface structures on dropwise condensation heat transfer under two different environments: pure vapor and an air-vapor mixture. Our experimental results show that, although convex structures enable faster droplet growth in an air-vapor mixture, the same structures impose the opposite effect during pure vapor condensation, hindering droplet growth. We developed a model for each case to predict the heat flux distribution along the structured surface, and the model shows reasonable agreement with experimental results. This work demonstrates that the effects of geometric features on dropwise condensation are not invariable but rather dependent on the scenario of resistances to heat and mass transfer in the system. The fundamental understanding developed in this study provides useful guidelines for condensation applications including power generation, desalination, dew harvesting, and thermal management.