[4] He was then recruited by his mentor, Eli J. Glatstein[5] to complete a separate residency in radiation oncology at the NIH between 1991 and 1994, where he eventually attained the rank of commander in the U.S. Public Health Service Commissioned Corps between 1989 and 1995.

[4] In 1996, Hahn joined the University of Pennsylvania School of Medicine as a radiation and medical oncologist as well as a researcher funded by the National Institutes of Health (NIH).

During this period as an active clinician, Hahn specialized in treating thoracic, sarcomatous, and genitourinary cancers, as well as the use of photodynamic therapy for the treatment of pre-invasive and invasive malignancies.

[19] On March 1, 2020, Vice President Mike Pence and Health and Human Services Secretary Alex Azar announced the addition of Hahn to the White House Coronavirus Task Force.

[28] Hahn has said that Trump "has asked FDA to cut back red tape and try to speed medical products into the hands of providers, patients and American consumers.

"[27][29] The FDA was criticized in the beginning stages of the pandemic in the U.S. for moving slowly and bureaucratically in approving COVID-19 tests developed by laboratories, prompting concerns from academic medical centers and others.

[30] After controversy over the hydroxychloroquine and plasma authorizations, Hahn transferred two political appointees from the top public relations and communications posts at the agency, replacing them with career civil servants on an acting basis.

Emily Miller, the agency's top spokesperson, who had no prior medical or science experience, was removed on August 28 after just two weeks on the job.

[31] On September 2 John "Wolf" Wagner, a close ally of chief DHS spokesman Michael Caputo, was removed from his post as FDA's associate commissioner for external affairs after serving for two months.