The microstructure and mechanical hardness of Inconel 718 (INC718) hexagonal honeycomb cellular structure manufactured by selective laser melting (SLM) was studied in this work. Non-heat-treated SLM-produced samples with cell wall thicknesses of 0.4, 0.6, and 0.8 mm were studied. The hardness was measured using MTS Nanoindenter at different temperatures and strain rates. At room temperature, continuous hardness measurements through a penetration depth of 2 µm using three different strain rates (0.02, 0.05, and 0.08 s−1) were performed. At the temperatures 100 and 200 °C, single hardness measurements at eight different maximum loads were performed. Using scanning ion microscopy (SIM), the grain size was found to change significantly as the cell wall thickness reduced from 0.6 mm to 0.4 mm compared to the change from 0.8 mm to 0.6 mm. A similar trend in mechanical hardness reduction was observed in the three samples. The microstructure, hardness, and strain rate sensitivity displayed anisotropy in properties between the planes parallel and perpendicular to the build direction. A model describing the temperature, strain rate, and indentation size effects on hardness developed by the second author was modified and used to evaluate the intrinsic material length scale used in gradient plasticity theory.