This letter proposes a novel manually actuated mechanism to control the position and posture of a passenger for preventing accidents due to stroller tipping while ascending or descending steps. This mechanism is designed as a four-bar linkage, which uses the rotational torque applied to the stroller’s handle by the user’s wrist motion while tilting to change the position and posture of the seat. Link parameters were determined to ensure sufficient posture transition while ascending or descending steps based on kinematic and dynamic analyses of the stroller. By using the rotational motion of the user’s wrist relative to the stroller body as an input, the position and posture of the seat could be controlled to maintain stability. We present the detailed design process of the mechanism using a simulation environment and the results of validation experiments conducted using a prototype.