Koh served as Commissioner of Public Health for the Commonwealth of Massachusetts (1997–2003) after being appointed by Governor William Weld.

During his service, the state saw advances in areas such as tobacco control, cancer screening, bioterrorism response after the September 11 attacks in 2001 and anthrax, health issues of the homeless, newborn screening, organ donation, suicide prevention and international public health partnerships.

In recognition of his national contributions to the field of early detection and prevention of melanoma, the Boston Red Sox designated him a “Medical All Star” (2003) which included the ceremonial first pitch at Fenway Park.

Koh and his wife, Dr. Claudia Arrigg, are the parents of three children, Steven (Korean name 고원영),[5] Daniel, and Katherine.

[7] Daniel served as Chief of Staff to former Boston mayor and later Secretary of Labor Marty Walsh from 2014 to 2017 and from 2021 to 2022, and was previously elected as a Member and Vice Chair of Andover Board of Selectman after very narrowly losing the 2018 preliminary election for Massachusetts's 3rd congressional district to Lori Trahan.

[8] His brother is Harold Hongju Koh, the former Legal Adviser of the Department of State and Dean of Yale Law School.