Morgan said 64,000 people had been apprehended or deemed inadmissible in August -- down from 82,000 in July and 144,000 from the peak in May.
Morgan credited President Donald Trump's "unprecedented network of initiatives," including the push to have Mexico conduct more enforcement on the border, with securing that 56% drop since May.
The August numbers show a consistent decline in arrests since the spring, and come as the Trump administration has pushed forward numerous controversial policies intended to help stem the flow of migrants arriving at the southern border.
The number of crossings has traditionally dropped during the summer months, but Morgan said the reduction "has nothing to do with seasonal trends."
But while citing Mexico's deployment of the national guard and acceptance of asylum seekers waiting for their immigration proceedings, Morgan said that support may not last into next year.
"I am concerned whether the government of Mexico" and Northern Triangle countries of Guatemala, El Salvador and Honduras "are going to be able to sustain the level of commitment that they have," he said.
Acting Homeland Security Secretary Kevin McAleenan, appearing alongside President Donald Trump at the White House last Wednesday, praised "continued partnership with Mexico" and regional support from Central American countries to combat irregular migration as well as enhancing security at the border.
In an interview with CNN, acting Immigration and Customs Enforcement Director Matt Albence also cited help from Mexico as a factor in the decrease in border apprehensions.
"A lot of what we're doing and a lot of the success we're seeing is a result of our foreign partners, especially with what Mexico has done with regard to putting national guard on their southern border as well as on their northern border," Albence said. "Those efforts need to be sustained in order to keep these numbers at a low level, especially in the absence of action by Congress."
Over recent months, the administration has expanded its program of returning some asylum seekers to Mexico for the duration of their immigration proceedings, introduced a rule that would dramatically limit the ability of Central American migrants to claim asylum in the US and moved to scrap a settlement that put a 20-day limit on family detentions.
Still, administration officials have pointed toward regional support as another reason for the drop in apprehensions.
"I'm not saying we are out of the woods just yet, but it speaks out where we were to where we are right now," a senior Border Patrol official said, noting the drop in apprehensions in the Rio Grande Valley -- which has been the busiest sector in recent years -- as an example. "I would give more credit to Mexico than anything else."
The acting CBP commissioner also touted the President's border wall. "This wall is absolutely needed to safeguard our southern border," Morgan said.
In his letter, Esper told Congress that he has "determined that 11 military construction projects along the international border with Mexico, with an estimated total cost of $3.6 billion, are necessary to support the use of the armed forces in connection with the national emergency."
Defense Department officials say 127 military construction projects are being put on hold in order to use the $3.6 billion to fund building 175 miles of southern border wall.