He had served as President Obama's special envoy to Sudan and South Sudan for two years, "building on a long legacy of dedicated service in the United States government," the White House said in a statement Monday.
"Princeton has done a tremendous job in helping to realize the promise of an independent South Sudan, and working toward the international vision of Sudan and South Sudan living side by side in peace," Obama said in the statement. "The people of Sudan and South Sudan, who have suffered so much, have the opportunity to seize a brighter future because of Princeton's efforts to urge both sides to put the interests of their people first."
No reason was given for Lyman's departure and no replacement was announced, the Sudan Tribune reported.
During his tenure, Lyman mediated with Sudan and South Sudan to resolve issues such as border questions, oil, the status of Abyei region and insurgency near South Sudan borders.