How to accurately characterize the modulus of living cells at the whole-cell level with a well-defined measurement geometry and precise mathematical modeling of viscoelastic relaxation is an ongoing challenge in biophysics and mechanobiology. Here, we report combined atomic-force-microscopy (AFM) measurements of stress relaxation and indentation force for 10 cell types ranging from epithelial, muscle, and neuronal cells to blood and stem cells, from which we obtain a unified quantitative description of the compressive modulus E(t) of individual living cells. The cell modulus E(t) is found to have an initial exponential decay at short times t followed by a long-time power-law decay together with a persistent modulus. The three components of E(t) at different timescales thus provide a digital spectrum of mechanical readouts that are closely linked to the hierarchical structure and active stress of living cells. This work provides a reliable experimental framework that can be utilized to characterize the mechanical state of living cells and investigate their physiological functions and diseased states.