The addition is effective on Friday and brings the board to a total of 15 members.
Mullen, 66, is a retired Navy Admiral who was the main military adviser to Presidents George W. Bush and Barack Obama from 2007 to 2011.
GM CEO Dan Akerson says Mullen has experience in change management, strategic planning and technical innovation.
In the Navy, Mullen commanded three ships, an aircraft carrier battle group and a fleet, GM says in a statement. He was chief of naval operations from 2005 to 2007.
Like Akerson, Mullen is a graduate of the U.S. Naval Academy. Mullen also finished the Advanced Management Program at Harvard Business School.