The Governor of Hong Kong Cecil Clementi, who had a profound interest in the Chinese language, decided to establish a school using Chinese as the medium of instruction. In 1926, Clementi Government Vernacular Middle School was founded. Clementi Government Vernacular Middle School is the first government school to use Chinese as the medium of instruction. Li King Hong, the then-Chinese language inspector of schools, was appointed the first principal of the school.
During World War II, the school suspended its operations under the Japanese Occupation of Hong Kong. After the war, the school was renamed Government Vernacular Senior Middle School . It was renamed again in 1951 to Clementi Middle School and in 1988 to Clementi Secondary School, after the governor.
The school has been relocated several times and its current site at Fortress Hill Road was established on 23rd September 1961.
The school Principal now is Mr. Li Kwok-kai , who succeeded Mrs. Tang, Wai Ling Chan upon her retirement in 2006.
* , Biochemist, Dean of United College, The Chinese University of Hong Kong,Chairman and Professor of Biochemistry at The Chinese University of Hong Kong,.
* Leung Pak-wah Edwin,Professor of Asian Studies at Seton Hall University and medalist of Ellis Island Medal of Honor.
* Wong Cheuk-Yin, Physicist, former chairman of Overseas Chinese Physics Association, U.S.A.
*,Biologist, Chair Professor, Department of Biology, Hong Kong Baptist University.
Politicians, government officials, and legislators
* Lee Ming Kwai, Former Commissioner of Police of Hong Kong.
* Leung Kwok-hung, a political activist in Hong Kong, China.
* Tsang Yam Pui, Former Commissioner of Police of Hong Kong.