Provided are an organic light emitting display device and a method of manufacturing the same. The organic light emitting display device includes a thin-film transistor (TFT), which includes an active layer, a gate electrode, and source/drain electrodes; an organic electroluminescent device electrically connected to the TFT and includes a pixel electrode formed on the same layer as the gate electrode, an intermediate layer including an organic light emitting layer, and a counter electrode that are stacked in the order stated; and a capacitor, which includes a bottom electrode, which is formed on the same layer and of the same material as the active layer and is doped with an impurity; a top electrode formed on the same layer as the gate electrode; and a metal diffusion medium layer formed on the same layer as the source/drain electrodes and is connected to the bottom electrode.