She attended Harvard College, where she was awarded a Harry S. Truman Scholarship and graduated in 1990 with a Bachelor of Arts, magna cum laude, in social studies.

She then attended Harvard Law School, where she was a semi-finalist in the Ames Moot Court Competition and graduated in 1993 with a Juris Doctor.

From 2000 to 2002, she worked as a senior associate at Wilson Sonsini Goodrich & Rosati, a Palo Alto, California law firm.

From 2002 to 2008, Koh worked as a litigation partner at the Silicon Valley office of the law firm McDermott Will & Emery representing technology companies in patent, trade secret and commercial civil matters.

[15] In 2020, Koh presided over a case in which a coalition of local governments, activist groups, and American Indian tribes sued the Commerce Department over the Trump administration's intention to end the 2020 Census early.

President Biden nominated Koh to the seat to be vacated by Judge Richard Paez, who had announced his intent to assume senior status upon confirmation of a successor.

[28] Koh was unanimously rated "well qualified" for the circuit judgeship by the American Bar Association's Standing Committee on the Federal Judiciary.

[41] On November 13, 2023, Koh was in a 7-4 majority that temporarily blocked Idaho's abortion ban due to its lack of exceptions for medical emergencies.

[43] On June 27, 2024, the Supreme Court dissolved the stay in Moyle v. United States, reinstating the 9th circuit's injunction against the ban.

On January 11, 2024, Koh struck down a San Francisco ordinance that prohibited homeless people from "sleeping, lodging or camping on public property.

"[44][45] Koh is married to Mariano-Florentino Cuéllar, the president of the Carnegie Endowment for International Peace and a former justice of the California Supreme Court and professor at Stanford Law School.