Washington has served as the Lions' secondary coach since 2008 after retiring as a player.
"Mark is one of our league’s best young coaches and I am very excited to have him accept the position of defensive co-ordinator," Lions coach Mike Benevides said in a statement. "He's a leader, a winner and he's demonstrated that he is more than ready for this next step in his coaching career."
Washington joined the Lions as a free-agent defensive back in 2003. He spent five seasons with the Lions, winning a Grey Cup with the team in 2006.
He started his CFL career in Montreal in 1997, where he played for six seasons and won a Grey Cup in 2002.
He began his pro career with the Barcelona Dragons of NFL Europa, where he helped his team to the 1997 World Bowl.
"I am grateful for the opportunity Mike has given me and I look forward to the challenge of helping us win a championship," Washington said in a statement. "We have a lot of talented players on this side of the ball and I expect great things from them in 2014."