The demands for effective thermally conductive dielectric composite materials continue to increase due to high speed and low-loss signal propagation required in microwave frequency applications. Composites of polytetrafluoroethylene (PTFE) and boron nitride are promising for these applications. Here, for the first time, recent generation surface-engineered boron nitride nanobarbs (BNNB) material was evaluated at a low filler content in PTFE to achieve the desirable high thermal conductivity and low dielectric loss requirements which are typically difficult without a strong filler–matrix interaction and high filler loading. Composites of modified BNNB and modified PTFE were also fabricated and evaluated. These composites exhibited higher thermal conductivity (1.2–1.3 W/mK) with a lower concentration of boron nitride nanoparticles than those reported in the literature. The data were analyzed by theoretical models and interfacial thermal resistance was the lowest for the composite made from the modified PTFE and modified BNNB. Additionally, the composites displayed excellent dielectric properties including a dielectric constant ≈2.3 and loss tangent of less than 0.005 at frequencies of 1–3 GHz. These results indicate that such thermally conductive and low-loss dielectric composites have significant potential as materials for thermal management and dielectric components in microelectronics, 5G, and microwave devices.