He was employed as an architect with I.M. Pei & Assocs, Manhattan, New York from 1959 to 1964. Made stateless by Japan in 1947, Ku acquired United States citizenship in 1959 and Korean citizenship in 1964. He married Julia Mullock on 25 October 1959 at St George's Church in New York and they adopted a daughter, Eugenia Unsuk. After the fall of Syngman Rhee, he returned to Korea in 1963 with the help of the new president Park Chung-hee, moving into the new building in Nakseon Hall, Changdeok Palace with his mother and wife. He lectured on architecture at Seoul National University and Yonsei University and also managed his own airline, Shinhan. When that went bankrupt in 1979, he went to Japan to earn money. In 1982, his family forced him to divorce his wife because she was sterile; his mother died in 1989. He started living with a Japanese astrologer, Mrs. Arita. In November 1996, he decided to reside permanently in Korea.
Death
Restlessly going back and forth between Japan and Korea, he eventually died of a heart attack, at the age of seventy-five, on 16 July 2005 at the Akasaka Prince Hotel, the former residence of his parents in Tokyo, Japan. His funeral was held on 24 July 2005 and his posthumous title decided as "Prince Imperial Hoeun of Korea" by the Jeonju Lee Royal Family Association. Yi Ku didn't have an heir. According to the Jeonju Lee Royal Family Association, Yi Won, Yi Ku's first cousin once removed, was appointed as the heir to him. Yi Ku already considered to adopt heir for the clan and Yi Won was considered; after meeting Yi Won several times, he was satisfied about the foreign language abilities of his cousin and allowed Won to be his successor. As of 10 July 2005, merely less than a week before his death, Yi Ku met the chairman of the association, Lee Hwan-ey, for the one last time, and Yi Ku formerly signed to adopt Yi Won as his heir. Despite that Yi Ku passed away soon afterwards, the Jeonju Lee Royal Family Association held a meeting for the adoption legitimacy in 21 July, and in the next day, 22 July 2005, Yi Won was officially recognized by the association to be the successor of late Yi Ku.
