Additive manufacturing of aluminum alloys is considered a promising layer-wise manufacturing method which can produce lightweight critical automotive/aerospace/military components with enhanced physical and mechanical properties. This paper aims at assessing the correlation between microstructure and small-scale characteristics of an additive manufactured AlSi10Mg alloy in the as-printed and heat treated conditions. Depth-sensing nanoindentation testing, as a non-destructive, robust, and convenient testing approach, along with microstructural assessments, using optical microscopy and scanning electron microscopy, were employed to compare the nano-hardness of the printed (selective laser melting method) and the heat treated (age-hardening) materials. Considering the distance from the build plate, a gradation in the cooling rate, and therefore the microstructure, is expected which directly affect the nano-hardness gradient along the deposition direction. Results show a transition in the microstructure from cellular grains, with coral-like silicon fiber colonies, to fragmented/spheroidized eutectic silicon particles upon the heat treatment. Unlike conventionally manufactured AlSi10Mg alloys, upon aging heat treatment in the additive manufactured AlSi10Mg alloy, the nano-hardness is decreased which is mainly contributed to stress relief, elimination of solid solution strengthening, and silicon spheroidization phenomena. These are considered in detail in the current paper.