The Young’s modulus of multilayer films containing cellulose nanocrystals (CNCs) and polyethyleneimine (PEI) was measured using a buckling-based method and compared to analogous films containing nanofibrillated cellulose (NFC) and PEI . For films 61 nm to 1.7 μm thick, the Young’s modulus was constant but strongly dependent on relative humidity. Films were stiffer at lower relative humidities, with modulus values of 16 ± 5, 12 ± 1, and 3.5 ± 0.3 GPa at 30%, 42%, and 64% relative humidities, respectively. CNC/PEI films had larger elastic moduli than NFC/PEI films. Both types of nanocellulose multilayer films showed the same modulus dependence on relative humidity over the range studied. Results suggest that ambient water might have an even more pronounced role in nanocomposites than in traditional natural fiber-reinforced composites. This straightforward buckling-based method has quantified mechanical properties and provided a useful comparison between CNC and NFC films. Furthermore, it qualitatively assesses that the components in the composite film are highly compatible and that the hydrophilicity and hygroscopicity of cellulose and PEI combined do not allow for the full mechanical potential of crystalline cellulose nanoelements to be exploited. This work is one approach toward finding dependable methods to characterize nanocellulose, specifically cellulosic thin films, which is increasingly important as we extract nanocellulose from wood, plants, algae, bacteria, and animals and enter a new age of cellulose materials.