WASHINGTON—President Donald Trump’s personal assistant, John McEntee, was escorted out of the White House on Monday, two senior administration officials said. The cause of the firing was an unspecified security issue, said a third White House official with knowledge of the situation.

White House press secretary Sarah Sanders declined to comment saying, “We don’t comment on personnel issues.” Mr. McEntee didn’t return a call seeking comment.

Tuesday morning, the Trump presidential campaign issued a statement saying that Mr. McEntee will join the 2020 effort as a senior adviser for campaign operations.

Mr. McEntee was one of the longest-serving aides to Mr. Trump, dating back to the earliest days of the campaign when some of the only aides around the then-candidate included Jared Kushner, the president’s son-in-law and senior adviser; Stephen Miller, the president’s policy director; White House communications director Hope Hicks, who announced her resignation two weeks ago; and Dan Scavino, who is the White House director of social media.