The U.S. State Department's special representative for Afghanistan and Pakistan is leaving his position this month.
Seventy-two-year-old James Dobbins, a veteran diplomat, came out of retirement last year to serve in the post.
U.S. Secretary of State John Kerry said Dobbins has "a lot to be proud of," having been at the "forefront" of U.S. efforts in Afghanistan and Pakistan.
Dobbins' departure comes as Afghanistan is struggling to determine the results of its presidential election, a process that has been undermined by fraud allegations.
In an interview with "The New York Times," Dobbins called the election impasse "serious" and said it could "present a real danger of a division."
But he said he is "hopeful" of an acceptable resolution to the crisis in the next few weeks, calling a clear result "probably most important variable for Afghanistan’s future."
Dobbins will be replaced by his deputy, Daniel Feldman.