WASHINGTON — Dr. Anthony Fauci, the nation’s top infectious disease expert who became a household name — and the subject of partisan attacks — during the Covid pandemic, announced Monday he will depart the federal government in December after more than five decades of service.

Dr. Fauci, who serves as President Biden’s chief medical adviser, has been the director of the National Institute of Allergy and Infectious Diseases and chief of the NIAID Laboratory of Immunoregulation. He was a leader in the federal response to HIV/AIDS and other infectious diseases even before the coronavirus hit.

“I will be leaving these positions in December of this year to pursue the next chapter of my career,” Dr. Fauci said in a statement, calling those roles “the honor of a lifetime.”

Dr. Fauci became the face of the government response to Covid as it hit in early 2020, with frequent appearances on television news and at daily press conferences with White House officials, including President Trump.

As the pandemic deepened, Dr. Fauci fell out of favor with Mr. Trump and his officials when his urgings of continued public caution clashed with the former president’s desire to return to normalcy.

Dr. Fauci found himself marginalized by the Trump administration, increasingly kept out of major decisions about the federal response, but he continued to speak out publicly in media interviews, advocating social distancing and face coverings in public settings before the rollout of the Covid vaccines.

He was also the subject of political attacks and death threats and was granted a security detail for his protection.

When Mr. Biden won the White House, he asked Dr. Fauci to stay on in his administration in an elevated capacity.

Dr. Fauci said despite retiring from federal service he planned to continue working. “I want to use what I have learned as NIAID Director to continue to advance science and public health and to inspire and mentor the next generation of scientific leaders as they help prepare the world to face future infectious disease threats,” he said.

Mr. Biden praised Dr. Fauci in a statement, saying, “Whether you’ve met him personally or not, he has touched all Americans’ lives with his work. I extend my deepest thanks for his public service. The United States of America is stronger, more resilient, and healthier because of him.”