The role will represent the first significant item in the vice president's portfolio, and her involvement has the potential to elevate the issue within the White House and broader administration.
Calling her "the most qualified person to do it," Biden said Harris would "lead our efforts with Mexico and the Northern Triangle" -- El Salvador, Honduras and Guatemala -- which, he said, "are going to need help in stemming the movement of so many folks -- stemming the migration to our southern border."
"She is going to be focused on overseeing our diplomatic efforts, working closely with these nations to look at the issues of migration and their own enforcement on their own borders," an official said. "More broadly, though, she's going to be working to implement a long-term strategy that gets at the root causes of migration."
Harris said Wednesday "there's no question that this is a challenging situation" and that she planned to engage with foreign leaders, civil society and the private sector.
"I also look forward to working with members of Congress who I think share our perspective on the need to address root causes for the migration that we've been seeing," she said Wednesday at the White House, before she and Biden discussed immigration with the secretaries of homeland security and health and human services, as well as other advisers.
Until now, Biden had not tasked Harris with a similar, high-profile issue she could make her own.
The president said Harris' experience dealing with organized crime and human rights issues as California's attorney general would prove useful.
"She's leading the effort, because I think the best thing to do is to put someone who, when he or she speaks, they don't have to wonder about, 'Is that where the President is?'" Biden said.
"When she speaks, she speaks for me," he said. "Doesn’t have to check with me. She knows what she's doing."
Harris will also work with Cabinet members like the secretary of state, as well as the administrator of the United States Agency for International Development and the U.S. special envoy for the Northern Triangle, an official said.
She is stepping into the role as the Biden administration struggles to balance its message that the U.S.-Mexico border is "closed" with its stated desire to find safe places for the minors who keep crossing the border unaccompanied by their parents. Federal facilities on the southern border have become overwhelmed with those minors.
In 2019, when the Trump administration was in the midst of enacting hard-line immigration policies, it also found U.S. facilities overflowing with migrants. That summer, President Donald Trump dispatched Harris' predecessor, Vice President Mike Pence, to tour U.S. Border Patrol facilities accompanied by cameras that captured packed, unsanitary conditions.
The officials declined to provide any specific travel Harris may be planning or calls she may have in the future, although an official did note that this morning the vice president spoke with the special envoy and other members of a U.S. delegation currently visiting Mexico.
On Monday, a reporter asked Harris if she planned to travel to the border -- neither she nor Biden have done so since taking office.
"Not today," Harris said, laughing. "But I have before and I'm sure I will again."