In 1906, he spent six months learning English at the Yeonghaksuk (英學塾) in Changpyeong, where his in-laws lived, together with Song Jin-woo (宋鎭禹).

In the same year, he acquired Kyungsung Jiknyu Joint Stock Company and began a textile business.

He resigned from his position as principal of Central School in March 1918 to focus on business management, but facing decline, he began the establishment of Kyungsung Spinning.

[3] He continued his education with texts like the "Zizhi Tongjian," the teachings of Confucius and Mencius, and Chinese history.

Personally, he enjoyed reading historical texts like Sima Qian's "Records of the Grand Historian" and the "Three Kingdoms.

In July 1921, Kim was elected as a committee member for the inaugural meeting of the Joseon Industrial Conference.

In March 1932, he acquired the financially struggling Boseong Professional School and served as its principal until June 1935.

In March 1935, Kim was appointed as the director and board member of the Joseon Commemorative Book Publishing Office.

After liberation, Kim Seong-su was approached by patriotic activists to join the newly formed Korean Democratic Party but initially declined.

In August 1946, Kim Seong-su established Korea University, building upon the foundation of Bosung Professional School, thereby laying the groundwork for a national private educational institution.