With the development of directed energy weapons (DEWs) such as high energy lasers and high-power microwave (HPM) weapons, there is a growing concern regarding the unknown biological effects that result from directed energy exposure. In this study, we create a multiphysics computational framework to model HPM exposure (~1 GHz) and the resulting thermomechanical expansion of brain tissue. A finite-difference time domain (FDTD) approach is used to calculate the specific absorption rate (SAR) in a human head geometry resulting from transient exposure to a pulsed-modulated microwave field. The SAR values provide a 3D temperature field that is subsequently used as input into a finite element model (FEM) in order to observe the thermomechanical expansion of brain tissue. A computational model is used to examine the influence of various temperature gradients and pulse durations on the mechanical response of brain tissue. We show that a stress-focusing effect due to a rapid temperature increase results in brain tissue strains much larger than the initially applied thermal strains.