Esper is set to start as acting defense secretary Monday following the departure Friday of current acting Defense Secretary Patrick Shanahan, the Washington Post reported.
The transition at the Pentagon comes amid escalating tensions between the U.S. and Iran, following Iran's downing of a U.S. drone over international waters and the country's suspected involvement on attacks against oil tankers in the Gulf of Oman earlier this month.
President Trump had announced Tuesday that Shanahan was withdrawing from consideration for a permanent appointment to lead the Pentagon because of family matters. In that same message, the president revealed that Esper was his choice to succeed Shanahan.
"I know Mark, and have no doubt he will do a fantastic job!," the president wrote.
Esper has been secretary of the Army since November 2017. He served in the Gulf War, worked on Capitol Hill and has been a lobbyist for Raytheon, a defense contractor. His Washington experience far outweighs that of Shanahan, who was a Boeing executive.
After Esper was nominated to lead the Army in 2017, the former infantry officer was confirmed by the Senate in an 89-6 vote, the Hill reported.
There has been no permanent secretary of defense since James Mattis resigned last December. Under the Vacancies Reform Act, which says department secretary positions can have an acting head for only 210 days, Trump must nominate a defense secretary by July 30, the Washington Post reported.
If nominated, Esper will have to step down from his acting secretary role while the Senate considers his nomination.