Of significant importance to the operation of electronic devices is the thermal characteristics of both the device in question and the operating environment of these devices. Efficient and reliable operation depends on holistic understanding of heat generation within the devices and the thermal contributions from the environment, this is of particular concern when the environments in which electronics systems are expected to perform is ever changing with continued human innovation and exploration. A critical barrier to the design of effective thermal dissipation architectures is the lack of accurate temperature information given by current thermal modeling efforts. To this effect, the first chapter of this thesis addresses the development of an ab initio phonon Monte Carlo (MC) framework for nanoscale, temporal temperature mapping of HEMT devices that leverages ab initio mode- and temperature-dependent phonon properties and combines ballistic phonon transport and interfacial phonon transmission to capture the thermal transport across multilayered structures. This framework is then applied to an AlN/GaN HEMT device and demonstrates that the decreased thermal conductivity with increasing temperature and decreasing size and the phonon reflection at AlN/GaN interfaces give a significantly higher temperature at the hot spot area than conventional technology computer-aided design (TCAD) prediction (at 7.61 W/mm and 8.43×10^5 W/mm3 heat flux, we see a ~80K difference), introducing secondary heating zones and exacerbating temperature discontinuities across material interfaces. The detailed temperature map with temporal evolution not only advances our understanding of the heat dissipation process in the HEMT devices but also reiterates the necessity of accurate phonon modeling for hotspot temperature prediction. This framework can be widely used to predict the temperature profile of various electronic devices even beyond HEMTs. The expansion of the use cases of electronic systems particularly in non-terrestrial applications that experience temperature maxima make design of electronic insulation and protection systems difficult. The second chapter addresses the use of high-temperature polymers for applications in extreme temperatures due to their mechanical flexibility and electrical insulating properties, albeit with their limited heat dissipation capability due to their intrinsically low thermal conductivities. Hexagonal boron nitride (hBN) is a chemically inert, thermally stable, and electrically insulative compound with a high thermal conductivity, making it an ideal candidate as a filler within a high-temperature polymer matrix to increase thermal conductivity. This study evaluates the effect of filler sizes and dispersion on thermal conductivity by producing homogenous composite samples using a combination of solvent mixing and resonant acoustic mixing (RAM). Samples were carefully characterized, including the spread of the size distribution, and observed that the smaller size centered around 5µm outperformed 30µm, in contrast to the conventional wisdom. Our thermal conductivity of hBN/ polytetrafluoroethylene (PTFE) composites at 30wt% is 2X higher than the literature values. Notably, the record-high value of 3.5 W/m-K at 40wt% was achieved with an onset of thermal percolation at 20wt%, attributed to optimized hBN dispersion that facilitates the formation of thermal percolation. Findings provide general guidelines to enhance the thermal conductivity of polymer composites for thermal management, ranging from power transmission to microelectronics cooling. The third chapter expands on our consideration of electrical insulation to be applied to unconventional environments. The extremely low frequency (ELF) and very low frequency (VLF) ranges are attractive for uses in high voltage (HV) systems in various unconventional environments such as space, subterranean and submarine applications. These HV systems require electromagnetic interference shielding (EMI SE) that allows for efficient heat dissipation and thermal management to protect and guarantee their efficient operation. The established mechanism of low frequency EMI SE in this ELF/VLF range is based on electrical conductivity with an increase in electrical conductivity corresponding to an increase in EMI SE presenting a barrier for direct use as an insulator. To this effect, our study developed an electrical insulator with enhanced thermal conductivity and ELF and VLF EMI shielding capabilities achieved via the layering of electrically insulating with electrically conductive high temperature polymer composites. We utilize MWCNT and h-BN composites leveraging the electrical conductivity of MWCNT and high thermal conductivity of h-BN within a PTFE matrix in a layered architecture that achieves a maximum thermal conductivity of over 1W/mK and EMI SE of over 80dB.