[1] A 33-year career diplomat and negotiator, Yun is an expert on Asia-Pacific affairs, having served multiple tours throughout the region and within the State Department.

Notable accomplishments included the diplomatic normalization of American relations with Myanmar,[8] the establishment of a US mission for ASEAN, and the inauguration of United States participation in the annual East Asian Summit.

As Ambassador, Yun emphasized stronger bilateral ties between the United States and Malaysia in all aspects: security, diplomatic, economic and people-to-people.

[14] During his 2014 visit, President Obama and Prime Minister Najib Razak signed the Comprehensive Partnership Agreement, pledging the United States and Malaysia to work closely together on security, business, education and technology issues.

[15] Yun also inaugurated the Oregon-Sabah Collaborative,[16] 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.

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.

[19] 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 vegetative state 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".

The COFA agreements were set to expire in 2023 for the first countries and in 2024 for Palau, raising US governments concerns about recent economic overtures from China as the deadline neared.