Trinity College was founded as a private secondary school in January 1958 at Melbourne Street, POS.
It was established through the efforts of the Very Rev. B. Vaughn, M.A., then Dean of the Cathedral of the Holy Trinity, and the Anglican Diocese to provide a sound secondary education for boys. This developed from the earlier project organized by the Cathedral Parish which showed the need for a secondary school.
In January 1959, upon application, the college became a government-assisted secondary school and thus became a public secondary school. Students are admitted in Form 1 on the basis of the Common Entrance Examination, and above Form 1 with the approval of the Ministry of Education based on their performance on transfer tests to other approved criteria.
From September 1987, girls have been admitted, but only in the sixth form.