A novel transition controller is presented to achieve seamless transition during the islanding and reconnection of microgrid with the main grid. The transition controller includes functionalities in supervisory level (operation mode management) and device control level (PCC operation control). Coordination between them follows the mechanism of the brain and the muscle: the operation mode management acts like a human brain to command the PCC operation control based on the request from DNO and the network conditions, and the PCC operation control executes the commands and informs the statue of PCC CB to the operation mode management. The developed transition controller is suitable for various microgrids and a sample microgrid is chosen to validate the design. The performance and effectiveness of the proposed microgrid transition controller were evaluated and demonstrated by a typical study case.