Mobile inverted pendulums (MIPs) need to be stabilized at all times using a reliable control method. Previous studies were based on a linearized model or feedback linearization. In this study, interconnection and damping assignment passivity-based control (IDA-PBC) is applied. The IDA-PBC is a nonlinear control method which has been shown to be powerful in stabilizing underactuated mechanical systems. Although partial differential equations (PDEs) must be solved to derive the IDA-PBC controller and this is a difficult task in general, we show that the IDA-PBC controller for the MIP can be derived solving the PDEs. We also formulate conditions which must be satisfied to guarantee asymptotic stability and show a procedure to estimate the domain of attraction. Simulation results indicate that the IDA-PBC controller achieves fast performance theoretically ensuring a large domain of attraction. We also verify its effectiveness in experiments. In particular control performance under an impulsive disturbance to the MIP are verified. The IDA-PBC achieves as fast transient performance as a linear-quadratic regulator (LQR). In addition, we show that even when the pendulum declines quickly and largely because of the disturbance, the IDA-PBC controller is able to stabilize it whereas the LQR can not.