Surface-enhanced infrared absorption spectroscopy (SEIRA) is attractive for molecular sensing due to its high sensitivity and access to molecular fingerprint absorptions. In this paper, we report on refractive index sensing of monolayer molecules in a spectral band outside the molecular fingerprint region. In a metagrating composed of a three-layer metal-insulator-metal structure, both propagating surface plasmon resonances (PSPs) and local surface plasmon resonances (LSPRs) are exited from free-space in a broad band of 3 to 9 µm, and their sensing properties are characterized. In response to a self-assembled monolayer of octadecanethiol (ODT) molecules, both PSPs and LSPRs exhibit redshifts in wavelength. The shifts of LSPRs are larger than those of PSPs, as originated from their stronger spatial confinement and larger field enhancement. Our proposed mid-infrared molecular sensor is immune to frequency variations of plasmon resonance and more tolerant to sample feature size variation.