The linear and nonlinear optical response of graphene are studied within a quantum-mechanical, full-band, steady-state density-matrix model. This nonpurtabative method predicts the saturatable absorption and saturable four-wave mixing of graphene. The model includes ${\tau }_1$ and ${\tau }_2$ time constants that denote carrier relaxation and quantum decoherence, respectively. Fits to existing experimental data yield ${\tau }_2<1\mathrm{fs}$ due to carrier–carrier scattering. ${\tau }_1$ is found to be on the timescale from $250\mathrm{fs}$ to $550\mathrm{fs}$ , showing agreement with experimental data obtained by differential transmission measurements.