Material heterogeneity is widely recognized for affecting the mechanical properties and behavior of concrete. One effective modeling approach to account for this factor is by representing the material at a mesoscale level. Rodrigues et al. [1] proposed a mesoscale model that considers the mortar matrix, aggregate, and interfacial transition zone phases, employing a Mesh Fragmentation Technique to represent potential crack paths. High aspect ratio interface elements are introduced between the regular elements in a finite element mesh. The regular elements behave elastically, while the interface elements are governed by a tensile damage model. This approach was successfully employed to study normal, high-strength and recycled aggregate concrete, where different fracture processes are observed due to the material mesostructure heterogeneity. In this study, the impacts of heterogeneity are investigated using a fully-coupled thermomechanical mesoscale model to evaluate the complex behavior of concrete when exposed to elevated temperatures. To achieve this, not only are the different mesoscopic phases represented, but a stochastic distribution is assumed for the mortar matrix material properties to reflect lower-scale heterogeneity [2]. The proposed model is validated against benchmark cases, demonstrating the model’s capability to simulate thermally induced cracks. Subsequently, a three-point bending beam exposed to elevated temperatures is simulated [3]. The experimental results are compared to the numerical results, showing good correspondence both quantitatively and qualitatively. Different levels of heterogeneity were investigated, demonstrating that the presence of such heterogeneity is a determinant factor for the decrease in mechanical properties and the propagation of cracking after exposure to elevated temperatures.