Yun joined the United States Foreign Service in 1985. In 2000, following tours in Hong Kong, Indonesia, Washington, DC, Paris, and Seoul, he was appointed Economic Counselor in the U.S. Embassy in Bangkok, managing economic, labor and environmental issues. In 2004 he attended the Asia Pacific Center for Security Studies, then took a 6-month assignment as senior adviser on the State Department Korea Desk. He returned to the U.S. Embassy in Seoul, in 2005 as the Minister-Counselor for Political Affairs, in charge of domestic, regional, and bilateral political issues. In 2009 Yun was posted back in Washington as Director of the State Department Office of Maritime Southeast Asia. The following year he was named Deputy Assistant Secretary for Southeast Asia, and in 2011 he was appointed Principal Deputy Assistant Secretary for the Bureau of East Asian and Pacific Affairs. During his time in EAP, Yun worked closely on President Obama's Asian rebalance policy, especially in Southeast Asia. Notable accomplishments included the diplomatic normalization of American relations with Myanmar, the establishment of a US mission for ASEAN, and the inauguration of United States participation in the annual East Asian Summit. For his work, the State Department has honored him with a Presidential Meritorious Service Award, three Superior Honors Awards, and nine Foreign Service Performance Awards.
Ambassador to Malaysia
Nominated by President Barack Obama to serve as Ambassador to Malaysia on July 23, 2013, Yun was confirmed by the U.S. Senate on August 1. As Ambassador, Yun emphasized stronger bilateral ties between the United States and Malaysia in all aspects: security, diplomatic, economic and people-to-people. During his three-year tenure, President Obama visited Malaysia twice, in 2014 and 2015. Prior to those trips, the last US President to visit Malaysia was Lyndon B. Johnson in 1966. During his 2014 visit, President Obama and Prime MinisterNajib Razak signed the Comprehensive Partnership Agreement, pledging the United State and Malaysia to work closely together on security, business, education and technology issues. Yun also inaugurated the Oregon-Sabah Collaborative, bringing together private citizens, local governments, universities and civil society from the two states to promote educational exchange, forest and wildlife conservation, and business ties. For his work in this area, Yun received the Individual Achievement Award from the Oregon Consular Corps in 2016.
North Korea Policy
On October 17, 2016, Yun assumed the office of United States Special Representative for North Korea Policy, heading all coordination and implementation of US policy toward North Korea, especially concerned with denuclearization. Concurrently, he held the office Deputy Assistant Secretary for Korea and Japan, managing relations with America's principal allies in developing a coordinated policy toward North Korea. Yun was the key diplomat securing the release of American student Otto Warmbier who had been imprisoned in North Korea for nearly a year and a half. In May 2017, Yun met secretly in Oslo with North Korean officials to gain diplomatic access to the four American prisoners held in Pyongyang. Learning on June 6 that Warmbier had been in a coma for the past 15 months, Yun flew with a medical team on an emergency mission to Pyongyang to secure his immediate release on "humanitarian grounds". He and the team returned Warmbier to his parents' care in the United States on June 12. While in Pyongyang, Yun also conducted a consular visit with the remaining American prisoners, the first since March 2016. As part of the agreement to release Warmbier, the North Koreans demanded $2 million as "repayment for medical expenses." Yun confirmed that he signed a pledge to pay that sum on the direct instructions of Secretary of StateRex Tillerson. However, Trump said in April 2019 that no money had ever been paid.