For instance, the president did not rely on press aides to explain his decision: He did that on his own. Mr. Trump told reporters gathered in the Oval Office on Tuesday for an unrelated event that Ms. Grisham “has been with me since the beginning” and added that she was well-liked by both White House officials and journalists.

“A lot of people wanted the job,” Mr. Trump said. “She’s here, she knows everybody, she actually gets along with the media very well.”

Mr. Trump said that Hogan Gidley, a White House spokesman who had the support of Jared Kushner, the president’s son-in-law and senior adviser, during the selection process, would remain in the West Wing as her top deputy. Ms. Grisham’s presence in the West Wing will also give Mrs. Trump a wider window into what goes on in a workplace where the president’s children have outsize influence, according to one senior aide.

Unorthodox boss aside, Ms. Grisham is still expected to institute changes to a communications office that has been in flux for two and a half years.

In recent weeks, White House officials have been debating internally whether to revive the daily news briefing as Mr. Trump heads into the thick of election season. A person familiar with the planning said on Tuesday that some version of a formalized news briefing was likely to be revived under Ms. Grisham. As press secretary, Ms. Sanders phased out the daily news briefings, a duty that has long been a primary function of the job, and conducted impromptu walk-and-talk meetings in the White House driveway.