Holbrooke, an assistant secretary of state under President Bill Clinton, characterized his work as helping to coordinate a "chaotic foreign assistance program" working with the military and others to "create a more coherent program."
"If our resources are mobilized and coordinated and pulled together, we can quadruple, quintuple, multiply by ten-fold the effectiveness of our efforts there," Holbrooke said Thursday.
While both countries are under one envoy, he said both would be respected for their individual histories and traditions.
Calling the region the "central front in our enduring struggle against terrorism and extremism," President Barack Obama said, "There is no answer in Afghanistan that does not confront the al-Qaida and Taliban bases along the border, and there will be no lasting peace unless we expand spheres of opportunity for the people of Afghanistan and Pakistan."
Holbrooke is a proven negotiator, having been instrumental in the Dayton Peace Accords for Bosnia, Obama said.
"He will help lead our effort to forge and implement a strategic and sustainable approach to this critical region," Obama said of Holbrooke. "The American people and the international community must understand that the situation is perilous and progress will take time."