Publicly, Mr. Mattis and other senior military leaders have refrained from criticizing the border mission.
Mattis estimated about 5,900 troops were participating in the border mission.
Leading up to the midterm vote on Tuesday, the military announced that the border mission would be called Operation Faithful Patriot.
Defense Department officials had raised the possibility this week of extending the border mission past its current Dec. 15 end date.
B5 Threats and Rights NEW BORDER MISSION -- Since the attacks, officials have been racing to tighten up the United States' sprawling border with Canada.
Correction: This story has been updated to say that the Army, not the Marine Corps, will deploy military police as part of the border mission.
This includes $72m for the 5,900 active-duty troops plus $13m so far for 2,100 national guard troops who have been performing a separate border mission since April.
Up to 2,100 National Guard troops have been deployed to the border mission in support roles after the president authorized the move in the spring, though it is unclear whether Trump is eyeing adding more guardsmen or using other military assets.
The military's border mission was slated to end Dec. 15, but the White House and the Department of Homeland Security had requested to continue using the troops as an additional bulwark against migrants from Central America entering the United States.
"We believe the deficiencies noted are systemic in nature and potentially represent a significant impairment of customs' border mission responsibilities in such areas as interdicting drug traffickers, smugglers and other law violators," the report said.
