This paper presents a novel approach to produce controlled tissue heating via the robotic control of a surgical laser’s focus. In clinical practice, physicians defocus a laser beam whenever they wish to change its effect from cutting to heating – e.g., to thermally seal a blood vessel. The overarching goal of this research is to create technology to help physicians control the heating and prevent thermal injuries. In the manuscript, we describe the synthesis and experimental verification of a nonlinear controller capable of tracking prescribed temperature profiles. The proposed controller was able to achieve tracking error (RMSE) < 2.5 °C for a target temperature of 50 °C. Furthermore, the controller was able to achieve consistent tracking performance in four different types of tissue (agar-based phantoms, and ex-vivo bovine liver, bone, and chicken muscle) without the need for re-tuning of its parameters, thus indicating its robustness to variations in the physical/optical properties of the tissue being irradiated.