[9] Harold was struck by polio at age six; he went through "two operations, leg braces, and endless rehabilitation" and as a result still walks with a limp.

From 1983 to 1985, Koh worked as an attorney-adviser to the Office of Legal Counsel (OLC) in the United States Department of Justice during the Reagan Administration.

[15] His students have included John Yoo, with whom he co-authored a paper on "Dollar Diplomacy/Dollar Defense: The Fabric of Economics and National Security Law."

A notable paper Koh wrote was a November 1990 legal brief challenging the first president Bush's contention that he could fight the Gulf War on his own authority.

Koh argued that "the Constitution requires the president to 'consult with Congress and receive its affirmative authorization — not merely present it with faits accomplis — before engaging in war.

'"[18] In 1992–93, he led a group of Yale students and human rights lawyers in litigation against the United States government to free Haitian refugees interned at Guantanamo Bay, Cuba.

[20][21] Other observers countered that during his tenure prominent conservatives have been appointed to the Yale Law School faculty, and noted that Koh served in both Republican (Reagan) and Democratic (Clinton) administrations.

"[22][citation needed] They concluded that "Dean Koh is one of the brightest legal minds of his generation, a credit to the profession we look forward to joining, and an able and effective public servant.

[31] Critics of this approach argue that citing foreign decisions as legal precedents threatens American sovereignty and "lends itself to manipulation.

"[32] Other commentators have observed that the "use of such nonbinding sources to bolster legal arguments is a central and uncontroversial tenet of the American judicial process.

In a March 2010 speech, Koh voiced his strong support for the legality of targeted killing by aerial drone strikes in Pakistan, Yemen, and other countries included by the U.S. government as being within the scope of the war on terror.

The State Department's legal adviser said that "U.S. targeting practices, including lethal operations conducted with the use of unmanned aerial vehicles (UAVs)", which the Obama administration has leaned on heavily in its efforts to eliminate al-Qaeda and other terrorist groups in Asia, "comply with all applicable law, including the laws of war", citing the principles of distinction and proportionality.

He said that the U.S. adheres to these standards, and takes great care in the "planning and execution to ensure that only legitimate objectives are targeted, and that collateral damage is kept to a minimum.

[45][46] Koh was also criticized by lawyer Jennifer Robinson, who represents activist Julian Assange, for addressing a letter to both her and her client.