Magneto-plasmonic sensors based on surface plasmon resonance have been studied considerably in recent years, as they feature high sensitivity and ultrahigh resolution. However, the majority of such investigations focus on prism-based sandwich architectures that not only impede the miniaturization of devices but also have a weak transverse magneto-optical Kerr effect (TMOKE) in magnitude. Herein, we theoretically demonstrate a magneto-plasmonic sensor composed of Au/Co bilayer nanodisk arrays on top of optically thick metallic films, which supports a narrow surface plasmon resonance (SPR) with a bandwidth of 7 nm and allows for refractive index sensitivities as high as 717 nm/RIU. Thanks to the high-quality SPR mode, a Fano-like TMOKE spectrum with a subnanometer bandwidth can be achieved in the proposed structure, thereby giving rise to ultrahigh sensing of merit values as large as 7000 in water. Moreover, we demonstrate a large TMOKE magnitude that exceeds 0.6. The value is 1 order of magnitude larger than that of magneto-plasmonic sensors reported. We also demonstrate that the behavior of TMOKE spectra can be controlled by tuning the geometrical parameters of the device including the diameter and thickness of nanodisk arrays. This work provides a promising route for designing magneto-plasmonic sensors based on metasurfaces or metamaterials.