With low-lifting capability taken into account, a robust guidance law for Mars entry vehicles with low lift-to-drag ratios, such as Mars Science Laboratory (MSL), is presented. Consider the nonlinear term in the drag dynamic equation and bounded disturbances as a lumped disturbance, and design a linear disturbance observer (DOB) to estimate it. With the consideration of the control input saturation, an innovative sliding surface and a virtual system are introduced to design the guidance law. Analyses of disturbance observer performance and Lyapunov-based transient performance are also presented. It is shown that the drag tracking error can be adjustable by explicit choices of design parameters. Simulation results confirm the effectiveness of the proposed guidance law.