Graphene nanostructures provide a compelling platform for near-unity absorption due to their tunable plasmonic response and large effective index change modulation. We numerically demonstrate a tunable Salisbury screen absorber in which graphene nanoribbons act as the resistive layer, combined with a low-index dielectric spacer and a metallic back reflector, to enhance radiative coupling to plasmonic resonators. An asymmetric Fabry–Pérot cavity model explains the absorption mechanism. Full-wave FEM simulations show up to 100% mid-infrared absorption, which is tunable via graphene’s chemical potential, nanoribbon width, and spacer thickness. For refractive index sensing, the device exhibits a high sensitivity of ∼2500 nm/RIU and a figure of merit of ∼32/RIU. The results highlight the potential of graphene-based absorbers for mid-infrared photodetectors and highly sensitive refractive index sensing applications.