Ms. Hicks had been considering leaving for several months. She told colleagues that she had accomplished what she felt she could with a job that made her one of the most powerful people in Washington, and that there would never be a perfect moment to leave, according to White House aides.

Her resignation came a day after she testified for eight hours before the House Intelligence Committee, telling the panel that in her job, she had occasionally been required to tell white lies but had never lied about anything connected to the investigation into Russia’s interference in the 2016 election.

She did not say what her next job would be, and it was unclear what the departure date is, although it is likely to be in the next few weeks. Ms. Hicks said she had “no words” to express her gratitude to the president, who responded with his own statement.