He was born in what was then the (village-designated) Yeoyu-ri, Koyang District, Gyeonggi Province (today a part of Yeouido Island, Seoul), in Japanese Korea.

In 1973–1974, he returned again to the United States, first as a fellow at Woodrow Wilson International Center (think tank) in 1973, and then at Harvard Law School in 1974.

In 1991, Lee was appointed South Korean Ambassador to the United Kingdom, also by Roh, and from 1994-95 Lee served as the 28th Prime Minister of the Republic of Korea under President Kim Young-Sam (though as Korea is a strong presidential system, the position of Prime Minister is subordinate to the presidency).

As Korea then had 47 seats to distribute on a proportional basis at the time, the high slot that the party gave Lee guaranteed that he would enter the National Assembly.

On March 24, 1998, President Kim Dae-Jung nominated Lee Hong-Koo to be South Korean Ambassador to the United States.

As Lee Hong-Koo was serving in a top advisory capacity for the Grand National Party, the decision to appoint him ambassador to the U.S. was seen to signal a change in the nature of Korean politics.

The Foreign Ministry explained that the nomination of a relatively important official of the previous government “shows our intention to pursue bipartisan foreign policy.” The concurrent nomination of Lee Sung-Soo as Unification Minister, also a former prime minister under the previous government, was seen to confirm this attitude.

Lee's tenure as ambassador was during the "Sunshine Policy", when apparently-improving inter-Korean relations led President Kim Dae-Jung to become a serious contender for the Nobel Peace Prize, which he won in October 2000.

In May 2002, Lee Hong-Koo founded the East Asia Institute (generally called "EAI" after its English acronym.

[6] Lee Hong-Koo served as the Chairman of the Board at EAI for exactly ten years, retiring from the post in May 2012,[7] the same month as his 78th birthday.