Cluster-assembled solids (CASs) formed by the self-assembly of monodispersed atomically precise monolayer-protected noble metal clusters are attractive due to their collective properties. The physical stability and mechanical response of these materials remain largely unexplored. We have investigated the mechanical response of single crystals of atomically precise dithiol-protected Ag29 polymorphs, monothiol-protected Ag46, and a cocrystal of the latter with Ag40 (formulas of the clusters have been simplified merely with the number of metal atoms). The Ag29 polymorphs crystallize in cubic and trigonal lattices (Ag29 C and Ag29 T, respectively), and Ag46 and its cocrystal with Ag40 crystallize in trigonal and monoclinic lattices (Ag46 T and Ag40/46 M, respectively). The time and loading-rate-dependent mechanical properties of the CASs are elucidated by measuring nanoindentation creep and stress relaxation. The obtained Young's modulus (Er) values of the CASs were similar to those of zeolitic imidazolate frameworks (ZIFs) and show the trend Ag29 T > Ag29 C > Ag40/46 M > Ag46 T. We have also studied the viscoelastic properties of all of the four CASs and found that the value of tan damping factor of monothiol-protected Ag46 T was higher than that of other CASs. The unusual mechanical response of CASs was attributed to the supramolecular interactions at the surface of nanoclusters. This observation implies that the stiffness and damping characteristics of the materials can be modulated by ligand and surface engineering. These studies suggest the possibility of distinguishing between the crystal structures using mechanical properties. This work provides an understanding that is critical for designing nanocluster devices capable of withstanding mechanical deformations. Copyright © 2020 American Chemical Society.