Thermal manikins are widely used to evaluate the thermal environment of automobiles and buildings, as well as the thermal characteristics of textiles. This study reported on the development and application of a new control of a thermal manikin with the open-source human thermoregulatory model JOS-3, expanding its use to transient conditions. The newly developed control utilizes the capability of JOS-3 to change personal parameters (e.g., age, sex, body composition). A case study with a local heating device was conducted in a climate chamber, both at steady-state with the conventional comfort mode control and at transient conditions with the JOS-3 control. Measurements were conducted at temperatures of 16, 19, and 21 °C, with the thermal manikin seated at a desk and dressed in a clothing ensemble corresponding to 0.8 clo. For the transient control, the physiological response of young and elderly individuals to local heating was compared. Results showed that the JOS-3 control was able to capture heating effects due to blood circulation at body parts that were not directly exposed to the heating device, such as the hands. The comparison of the predicted whole-body thermal sensation revealed that elderly individuals were not able to benefit from the local heating at 16 °C due to the cold sensation at the non-heated body parts. The case study demonstrated that the newly developed control could aid in expanding the scope of thermal comfort studies from an average person to a more diverse group of people.