The anharmonic behavior of phonons and intrinsic thermal conductivity associated with the umklapp scattering in monolayer MoS${}_2$ sheet are investigated via first-principles calculations within the framework of density functional perturbation theory. In contrast to the negative Grüneissen parameter ($\gamma$) occurring in low-frequency modes in graphene, positive $\gamma$ in the whole Brillouin zone is demonstrated in monolayer MoS${}_2$ with much larger $\gamma$ for acoustic modes than that for the optical modes, suggesting that monolayer MoS${}_2$ sheet possesses a positive coefficient of thermal expansion. The calculated phonon lifetimes of the infrared active modes are 5.50 and 5.72 ps for $E^{\prime }$ and $A_2^{\prime \prime }$, respectively, in good agreement with experimental results obtained by fitting the dielectric oscillators with the infrared reflectivity spectrum. The lifetime of the Raman $A_1^{\prime }$ mode (38.36 ps) is about seven times longer than those of the infrared modes. The dominated phonon mean free path of monolayer MoS${}_2$ is less than 20 nm, about 30-fold smaller than that of graphene. Combined with the nonequilibrium Green's function calculations, the room temperature thermal conductivity of monolayer MoS${}_2$ is found to be around 23.2 W m${}^{-1}$ K${}^{-1}$, two orders of magnitude lower than that of graphene.