Well-travelled Robinson, 46, who hails from Llandrindod Wells, spent five successful years in the United States leagues in charge of Canada outfit Vancouver Whitecaps.
For the last three years he has worked in Australia with Newcastle Jets and Western Sydney Wanderers but now returns stateside to work with Manchester United and England legend Rooney.
Rooney said: “Carl is an experienced MLS coach who has a great understanding of the league, both as a player and coach.
“He’s already added a lot of value to the coaching side and his knowledge and experience will be a great addition.”
Robinson added: “I’m proud to join DC United and be a part of the coaching staff for such a historic MLS club.
“I’m excited about the project that’s being led by Wayne Rooney and believe that the team can be successful this season.”