Mulvaney said that will require "bigger discussions for another day."
Trump's budget blueprint, which is being sent to Congress for review, proposes a $54 billion increase in defense spending, with cuts to foreign aid and domestic programs.
"This budget will be a public safety and national security budget," he told the National Governors Association on Monday. He said the budget will include an increase in "all spending for federal law enforcement."
Administration officials have stressed that the budget plan is merely a first draft and shell of what it will send Congress in a formal budget request next month.
ABC News' Alexander Nallin and The Associated Press contributed to this report.