John Sullivan, the U.S. ambassador to Russia, leaves Moscow to retire.

John J. Sullivan, the U.S. ambassador to Russia, left Moscow on Sunday and is retiring from public service after serving under five American presidents, the State Department said.

Mr. Sullivan oversaw the operations of the U.S. diplomatic mission in Russia during its most difficult period in decades — after President Vladimir V. Putin of Russia ordered a full-scale invasion of Ukraine in February, prompting the United States and partner nations to impose economic sanctions on Russia and give Ukraine weapons and humanitarian aid.

Mr. Sullivan, a Republican, served as deputy secretary of state under both the secretaries of state in the Trump administration, Rex Tillerson and Mike Pompeo, before being appointed ambassador to Russia in December 2019. He served as acting secretary of state in April 2018, before Mr. Pompeo began his tenure leading the State Department.

And before the Ukraine war, Russia and the United States had for five years engaged in retaliatory actions over diplomatic staffing. Mr. Sullivan told Politico in a written exchange in February that the number of employees in the U.S. embassy in Moscow had dropped to about 150 from about 1,200 during that period. Mr. Sullivan said there were entire floors of the embassy that were unoccupied, and row after row of empty cubicles and dark offices.

Mr. Sullivan agreed to stay on as ambassador when asked by President Biden after he took office in January 2021.

“It may be passé now, but I was taught that when a president asks an American to serve, only the most compelling excuse can justify a refusal with great remorse,” Mr. Sullivan said in the written interview. “And I had no excuse, because I love my job, and I love working with my colleagues at Embassy Moscow.”

The State Department said that Elizabeth Rood, a foreign service officer who became deputy chief of mission in Moscow in June, will serve as the chief of mission until a new ambassador is named and takes up the post.