The 31-year-old has not played for the club in almost a year, after being diagnosed with a heart ailment.
Johnson began his career at fellow MLS club FC Dallas, and eventually moved on to Fulham in 2008, although he struggled to make an impact at the Cottagers.
"Eddie had many outstanding accomplishments during his career and was a valuable member of the team during his time with DC United," DC United general manager Dave Kasper told the club's official website.
"He will be missed but always remembered as he moves on beyond the playing field."
Johnson scored 19 goals in 63 appearances for the USA national side and helped his country to win the Gold Cup in 2007 and 2013.