Recent improvements in thermoelectric (TE) materials have expanded the potential to use this technology to generate electricity from waste heat in a variety of applications. The performance of a TE generator improves when the temperature difference across the generator is as large as possible given the constraints associated with its application. This paper considers the use of a “thermal switch,” located physically between the heat source and the TE device, to modulate the heat flow through the TE device. A control schema is envisioned which permits heat to flow from the source to the TE device only when the source temperature is near maximum, yielding a higher time-averaged temperature drop across the TE and therefore a higher efficiency. A numerical model is used to evaluate the benefits of an active thermal switch in series with a TE generator relative to a baseline case defined by the absence of a thermal switch for both time-varying and constant heat inputs. The results demonstrate that modulating the heat flow through the TE device and maintaining the source temperature at a near constant maximal value is capable of improving the time-averaged TE device energy recovery efficiency. For some conditions, improved efficiencies of more than five times are realized. The requisite physical conditions necessary for achieving these improvements are also identified.