The results of a first-principles model for the equation of state of MgO from ambient to combined pressure-temperature conditions assumed for the earth's lower mantle are presented. Ab initio electron-gas pair potentials are used for the short-range interactions. The thermal contribution to the equation of state is determined from the quasiharmonic phonons calculated from the pair potentials. The model successfully reproduces the room-temperature compression curve measured to ~1 Mbar and the thermal expansion at zero pressure. The volume coefficient of thermal expansion (298 K) is calculated to drop to 30% and 18% of the zero-pressure value at 0.5 Mbar and 1.0 Mbar, respectively.