The school was opened by Mr. John Crawford and Sir Edward Coey in 1887. Mr. R.M. Jones M.A. opened the school to its first pupils. Larne Grammar School was set up as an all boys fee paying and boarding school. During the school's first 20 years, very few pupils attended, and the school was threatened to be closed on numerous occasions. The school temporarily closed between 1914 and 1918 due to the outbreak of World War One. In 1922, Larne Grammar School merged with Larne Girls' School, and a Preparatory Department was established. Larne Grammar began to grow, with more and more pupils joining the voluntary school. During the early 1970s, the school was extended to accommodate the growing number of pupils. 19 new classrooms, a sports hall and changing rooms were built to the rear of the school. During the leadership of Headmaster D.J. Thompson, the original school building was demolished and replaced by a new school in 1987. 22 new classrooms, a new sports hall,new staff offices and a new canteen were built, as well as a new atrium, reception and Headmaster Suite. The extension from 1972 remained, but was refurbished. The original assembly hall was retained and still remains today. In 2001, under the command of John Wilson, a new Technology and Design Department was established. The new building consists of two classrooms with workshops, offices and lavatories. The building has solar panels and provides all of the energy required to power the entire school. Furthermore, a new Music Department was constructed towards the rear of the building, with a large classroom, offices and tuition rooms.To celebrate the 125th Anniversary of Larne Grammar School, a new astro-turf hockey pitch, four tennis courts and a new pavilion were built. All classrooms have been fully refurbished during the 2012/13 school year, with new equipment being added such as Apple Computers, new desks and new chairs; all of which under the orders of new Headmaster Jonathan Wylie.