He previously served as the Acting Assistant Attorney General of the Office of Legal Counsel at the United States Department of Justice.

[3][2] In 2009, while on leave from his faculty position, Barron rejoined the Office of Legal Counsel as Acting assistant attorney general.

In 2010, he authored a secret memo which provided the legal foundation for President Obama's unprecedented decision to order a drone strike on Anwar al-Awlaki, an American citizen who was a radical Islamic militant living in Yemen.

[4] Barron's memo was described by The New York Times Editorial Board as "a slapdash pastiche of legal theories—some based on obscure interpretations of British and Israeli law—that was clearly tailored to the desired result.

"[5] A lawyer for the ACLU described the memo as "disturbing" and "ultimately an argument that the president can order targeted killings of Americans without ever having to account to anyone outside the executive branch.

[15] Barron is known for coauthoring with Martin S. Lederman a Harvard Law Review article titled "The Commander in Chief at the Lowest Ebb - Framing the Problem, Doctrine and Original Understanding,"[16] which was an attack of the advice given by the Office of Legal Counsel to President George W. Bush justifying Bush's use of executive power during the War on Terror.