The deal between Ashraf Ghani and Abdullah Abdullah broke a three-month deadlock over allegations of massive electoral fraud that plunged Afghanistan into crisis as US-led troops end their 13-year war against the Taliban.
Both Ghani and Abdullah claimed to have won the vote, but Ghani is widely reported to have come out ahead after a UN-supervised audit of all eight million ballot papers.
"Signing this political agreement helps bring closure to Afghanistan's political crisis, and restores confidence in the way forward," White House spokesman Josh Earnest said in a statement.
He said the deal "prioritizes the recognized needs of the Afghan nation ahead of politics or individual power."
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Under today's deal, Abdullah will nominate his choice for the new post of "chief executive officer" (CEO), which will be similar to prime minister -- setting up a tricky balance of power as Afghanistan enters a new era.
"This agreement marks an important opportunity for unity and increased stability in Afghanistan," Earnest said.
"We continue to call on all Afghans -- including political, religious, and civil society leaders -- to support this agreement and to come together in calling for cooperation and calm."
He spoke of a "new chapter in our enduring partnership with Afghanistan."
"These two men have put the people of Afghanistan first, and they've ensured that the first peaceful democratic transition in the history of their country begins with national unity," he added.
Speaking of an "enormous opportunity" for Afghanistan to emerge stronger after the agreement, the top US diplomat stressed that "elections are not the end."
"They must be the beginning, where Afghanistan and its people move forward on a reform agenda and make improvements to the electoral process," he added.