Both Mitchell and Holbrooke are experienced negotiators; Mitchell crafted peace agreements in Northern Ireland and Holbrooke mediated the Dayton Accords, bringing peace to Bosnia.
Biden called the envoys the administration's "two new powerful weapons."
The administration did not make any announcements about an envoy for Iran, another critical area for foreign policy.
As Clinton called for a new era in America's foreign relations, she said the Obama administration would not tolerate the divisiveness which she suggested had characterized the Bush years.
Clinton said she and the president want to send an "unequivocal message" that "this is a team, and you are the members of that team. There isn't anything that I can get done from the seventh floor or the president can get done from the Oval Office unless we make it clear we are all on the American team. We are not any longer going to tolerate the kind of divisiveness that has paralyzed and undermined our ability to get things done for America."
On Friday, Clinton will visit the United States Agency for International Development (USAID) to thank and greet employees there.
"They will be our partners," she said today.
Clinton also gave a nod to what has been known (both endearingly and perhaps not so) as "the building" -- diplomats who have been disgruntled in recent years for feeling they had been marginalized.
"I want you to think outside the proverbial box," she said. "I want you to give me the best advice you can. I want you to understand there is nothing that I welcome more than a good debate and the kind of dialogue that will make us better."
Those remarks brought big cheers from the assembled crowd.