Highly filled phthalonitrile nanocomposites, reinforced with both native and silane surface modified nano-BN were investigated for their mechanical and thermal properties. The nanocomposites were prepared by a solution blending technique followed by compression molding. The thermal conductivity was investigated as the core of the study along with a bending test, a morphological behavior, thermal stability investigations, and thermomechanical properties measurements. The highest thermal conductivity value of 4.69 W/m.K was obtained at the maximum nanofillers loading of 30 wt.% treated BN. A high resolution TEM confirmed that the exceptional enhancements in the thermal conductivity are related to the formation of an effective conductive path assured by the silane surface modified nano-BN. The bending test also revealed consequent improvements in both the strength and modulus as the amount of the nanofillers increased. The SEM investigations revealed positive changes in the surface morphology of the neat resin, with a rougher and homogeneous state of surfaces for the hybrids. Moreover, the thermal stability at the maximum fillers loading was found to be one of the best ever reported in the field of high performance polymer-based composites, with a 5 wt.% weight loss temperature of 551 °C.