Carbon-based materials are becoming a promising candidate for thermoelectricity. Among them, graphene shows limited scope due to its ultra-high thermal conductivity (𝜿). To develop graphene-based thermoelectric devices, reduction of 𝜿 is highly desired while maintaining reasonably high electrical conductivity (𝝈). Herein, multiwalled carbon nanotubes (MWCNTs) and carbon black (CB) fillers are added into few layered graphene (FLG) to produce all-carbon composites yielding ultra-low thermal conductivity (𝜿) desired for thermoelectric applications. The novel preparation method of pristine FLG realizes very low 𝜿 of 6.90 W m −1 K −1 at 1248 K, which further reduces to 0.57, 0.81, and 0.69 W m −1 K −1 at the same temperature for FLG + MWCNTs, FLG + CB, and FLG + MWCNTs + CB, respectively. As-prepared FLG composites also maintain reasonably high 𝝈, whilst the Seebeck coefficient shows over a factor of five improvement after the inclusion of carbon-based fillers. Consequently, the power factor (PF) is significantly improved. The ultralow 𝜿 is attributed to the increased thermal boundary resistance among graphene sheet boundaries. The realization of ultralow 𝜿 with simultaneous improvement in Seebeck coefficients and relatively small drops in 𝝈 with a facile and unique synthesis technique, highlight the potential of these composites.