An apparatus and a method for determining a mounting state of an electronic control unit (ECU) of an electronic controlled suspension (ECS) system can determine either a normal mounting state or a mounting failure of the ECU of the ECS system. The apparatus for determining the mounting state of the ECU includes: a signal processor receiving a damper velocity of each damper; an integrator calculating an integral of the damper velocity by integrating the damper velocity of each damper received by the signal processor per unit time; a comparator comparing the integral of the damper velocity calculated by the integrator with an actual stroke of each damper; and an index output unit outputting a mounting state of the ECU in the form of an index according to comparison results of the comparator.