The rapid advancement of lightweight and high-power electronic devices has led to increasingly aggravated issues of electromagnetic wave (EMW) interference and heat accumulation, which severely threaten the stability and service life of these devices. To address these challenges, we propose a simple and scalable mechanochemical strategy to activate hexagonal boron nitride flakes (BNFs) via liquid metal (LM), which results in outstanding EMW absorption and high thermally conductive capabilities. This facile modification approach avoids multistep reactions, high solvent consumption, and high energy input while simultaneously introducing abundant interfacial polarization centers, thus optimizing the EMW absorption performance of BNFs. Consequently, the optimized BNF@LM composites exhibit remarkable EMW absorption with a minimum reflection loss (RL_min) of −48.4 dB and a maximum effective absorption bandwidth (EAB_max) of 5.76 GHz. In addition, coupling the H-BNF@LM composites with aramid nanofibers (ANFs) can impart ANF-based films with good thermal conductivity and flexibility, making them suitable for flexible electronic devices. Typically, the H-BNF@LM/ANF films can reach a thermal conductivity of 0.54 W·m⁻¹·K⁻¹, as measured via the hot wire method, which is nearly 5 times greater than that of the ANF film (0.10 W·m⁻¹·K⁻¹). Moreover, the H-BNF@LM/ANF films can effectively attenuate incident EMW and exhibit remarkable flame retardancy, endowing them with strong adaptability to extreme environment.