McEntee was escorted out of the White House on Tuesday, according to the Wall Street Journal, as part of a "unspecified security issue".
People familiar with the matter said that McEntee had been able to obtain a security clearance earlier in Trump's presidency, and speculated that any issues that led to his firing were recent developments. A White House spokesman declined to comment on McEntee's firing.
"Mr. McEntee wasn't as well known as the others, but had been a constant presence at Mr. Trump's side for the past three years", the newspaper reported.
McEntee was one of Trump's longest-tenured staffers, having joined during the campaign.
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USA media reported his abrupt exit came about due to difficulties in obtaining a full security clearance.
It wasn't clear why McEntee, a well-liked figure in the White House, had been dismissed, and campaign aides weren't given a reason. He looks to have already landed on his feet - just minutes after news of his departure broke, it was announced that McEntee would be joining Trump's reelection team as a senior adviser.
"We need the help of proven leaders such as Katrina and John to promote the President's growing portfolio of achievements across the country", campaign manager Brad Parscale said in a statement.