A pulse-echo ultrasonic guided wave approach that characterizes the setting and hardening of early age mortar during the first twenty-four hours of hydration is presented. The method invokes the fundamental torsional mode on the free end of a cylindrical steel rod partially embedded in mortar and monitors the reflected signals. Both the reflection from the end of the rod and the reflection from the point where the waveguide enters the mortar are monitored. The evolution of the mortar properties is related to both the energy leaked into the surrounding mortar and the energy reflected at the entry point. Results show that the technique is useful for monitoring the development of the mechanical properties of varying water-cement ratios (w/c = 0.40, 0.50, and 0.60). In addition, the effects of chemical (accelerant and retardant) and mineral (silica fume and fly ash) admixtures on the guided wave behavior were also studied. Time of setting and compressive strength measurements have been performed on the various mortar mixtures. The change in signal strength of the end- and entry-reflection of the guided wave appears to be correlated to the mortar setting times and compressive strength, respectively. The ability of this method to only require access to one side of the specimen makes it attractive for the development of a system that monitors in-situ cementitious materials.