[2] Established in 1847 by the Reverend Henry Brereton, the first chaplain of Karachi, as a school for "English and Anglo-Indo children", it is the oldest private school in Pakistan and the second oldest in South Asia,[3][4] a member of the Winchester International Symposium and a former member of the Headmaster's Conference.

The class formed by the chaplain was at first small enough to be accommodated in these modest premises; however, the smooth running of this school over the next seven years was disturbed by rumours of Brereton not being a "good master" and his performance as a manager being unsatisfactory.

Looking into this matter, on 27 July 1854, the Commissioner of Sindh, Sir Bartle Frere, summoned a public meeting with a view to establishing an institution that provided 'good secular instruction to children of all sects'.

In this meeting, funds were collected through subscriptions to establish a school, a managing committee was appointed and rules were framed that later became the basis for the present Constitution of KGS.

Captain Thomas F. Dowden of the Royal Bombay Engineers was commissioned to make the architectural drawings for the new building.

The new school building was opened for boarders on 27 February 1875 by Sir William Merewether, Commissioner-in-Sind at that time.

Towards the end of 1914, construction began on the third story of the school, and students were temporarily taught in a building on Merewether Road, which was given free of charge.

[10] In the Junior Section, extracurricular activities available include sports, music, swimming, a school choir, scouts and girl guides, community service, etc.

It has entered competitions including LUMUN (Pakistan's largest international Model United Nations conference, hosting over 1200 delegates), where the school team won in 2008, 2009, 2012, 2015, 2019, 2020 and most recently, in 2021.

Karachi Grammar School also took part in Harvard Model United Nations 2012 in Beijing, China, winning the 'Best Large Delegation' award.

In 2022 the KGS Model United Nations Team founded KGSMUN their flagship MUN Conference.