Voltage and frequency regulations are main requirements for autonomous operation of an isolated microgrid with electronically interfaced distributed energy resource (DER) units. This paper presents a robust decentralized control design scheme for a multi-DER microgrid to enhance its voltage and frequency regulations. The proposed control scheme utilizes linear quadratic (LQ) decentralized control scheme to design controllers, which are robust to the uncertainty in the network and system nonlinearities. The effectiveness of the proposed controller is demonstrated through simulation on a test system, with complete nonlinear models, for large disturbances. It is found that the designed controller provides excellent performance during transient periods as well as restores the pre-disturbance steady-state operation within prescribed time frame.