Due to amalgamation of large-scale distributed generations (DGs), low frequency oscillation (LFO) becomes the primary concern in power systems. Therefore, it becomes paramount to design wide area damping controller (WADC) to provide sufficient damping to LFO modes. To design WADC, the availability of all the states is required. However, practical power grid is similar to black-box system and to design such controllers, reconstruction of all the states is necessary. In this effort, an observer based prescribed degree robust WADC is proposed for DG integrated system to damp the critical LFO modes. Furthermore, the WADC parameters are tuned by Nyquist robustness assessment and loop transfer recovery (LTR). Geometric approach is utilized to find the suitable control location in DG system and wide-area signals. The efficacy of proposed WADC is demonstrated on a complex 16-machine, 68bus system, and its performance is also compared to conventional optimal control method. From the simulation results, it is evident that the proposed WADC for DG furnishes enhanced damping towards critical modes for broad operating scenarios.