Calorimetry is the standard measurement method for monitoring the hydration of cementitious materials and retrieving their hydration degree. However, it is destructive and impractical for in situ measurements. Electromagnetic measurements are sensitive to the changes in the porosity and water content of materials, making dielectric techniques good candidates for characterizing the hydration of cementitious materials. Besides, they can be implemented in a noninvasive way by means of microwave reflectometry sensors. In this article, the hydration of a cement mortar is monitored using both calorimetry and microwave reflectometry at 800 MHz, over a 140 h period covering its hydration process. The measured calorimetry and dielectric data prove to be consistent with each other as well as with the phenomena at stake along hydration. Besides, the combination of this data is carried out to obtain the variations of the mortar dielectric permittivity as a function of the hydration degree. On the other hand, we develop a dielectric model for monitoring the mortar hydration degree. The latter, which is based on mixing equations, accounts for the age-dependent mortar dielectric properties due to the hydration process and porosity decrease experienced by cementitious materials, especially at an early age. This model is the first mechanistic approach to describe the dielectric properties of mortar during hydration. Thus, in order to retrieve the hydration degree, the used model implements mean-field homogenization based on the dielectric features of the mortar constituents. The combination of calorimetry and dielectric experimental data is compared to the dielectric model of hydration, showing good agreement.