Seat suspension plays a crucial role in attenuating low-frequency vibrations and improving overall riding comfort. This paper proposes a new nonlinear isolator for seat suspension using a novel high-static-low-dynamic stiffness with sliding mass inertia (HSLDS-SMI). The HSLDS-SMI seat suspension includes a negative stiffness structure and two secondary sliding masses. A multi-degree-of-freedom (MDOF) dynamic model is developed, comprising a simplified quarter-car model and a biodynamic human model. Various isolation performance indices are then considered and analyzed under two different road excitations using numerical methods. The impact of the sliding mass on ride comfort is examined under both road profiles. The findings demonstrate that the HSLDS-SMI seat suspension performs better than its two counterparts while ensuring the relative suspension deflection remains within acceptable limits.