David Barron was confirmed by a mostly party-line vote of 53-45. He will join the 1st U.S. Circuit Court of Appeals, which is based in Boston.
Barron was acting head of the Justice Department's Office of Legal Counsel in 2009 and 2010 when he authored memos explaining why the government had the constitutional authority to kill Americans in counterterrorism operations abroad, even if they were not on a battlefield.
In 2011, an unmanned U.S. drone in Yemen killed American-born Anwar al-Awlaki, who administration officials say was an al-Qaida leader. Officials acknowledge three other Americans killed by drones but say they weren't specifically targeted.
President Barack Obama's choice of Barron for the high-level job had drawn criticism from members of both parties. Some disagreed that the government has the power to kill Americans without a trial, while others objected to the administration's refusal to disclose the documents explaining its legal reasoning.
Republicans also said he is too liberal.
Barron's nomination gained steam after the administration revealed this week that it would not battle a federal appeals court order to release a censored version of one of his memos. That document has yet to be released.