The nucleus of the present Thornhill College commenced shortly after the Sisters of Mercy came to Derry in 1848. The Sisters started a private school for girls in Pump Street. The school was known as the Convent of Our Lady of Mercy School, Derry. Its first Scheme of Government under the then Ministry of Education was drawn up in February 1925. When Watt's Distillery closed in Derry, and the Estate at Thornhill, on the outskirts of Derry, became available for sale, the Sisters, with the kind assistance of Robert Boyle, a builder in Derry, completed the purchase of the property in 1929. The house was adapted to the needs of the nuns and of the boarders. The Sisters involved moved from Pump Street to Thornhill Convent along with the boarders, and the Convent of Our Lady of Mercy School was officially transferred and opened on 8 September 1932 with an intake of 120 pupils, and a staff of about seven Sisters and three lay teachers. After the Education Act, Northern Ireland in 1947, the number of pupils had increased rapidly as grammar school education became available to more and more children through the eleven plus examination. The school also had a change of name to Thornhill College, Convent of Mercy Voluntary Maintained Grammar School and various new buildings completed but these were insufficient. After many years of much effort, the present building was approved and opened in 2004 across the road form the old site in the new Thornhill College. At that time, the Sisters of Mercy withdrew from trusteeship, and gave it over to the Bishop and the Diocese of Derry.
Motto
The school motto is the Latin phrase "Adveniat Regnum Tuum", which translates as "Thy Kingdom come".
Academics
In 2009 Thornhill College became a Specialist School in Mathematics and Physics. The Specialist Schools Programme helps schools, in partnership with private sector sponsors and supported by additional Government funding, to establish distinctive identities through their chosen specialisms and achieve their targets to raise standards. In 2018 Thornhill College was ranked 17th out of 192 schools in Northern Ireland in terms of its A-level performance. A total of 84.3% of its students entered for A-Levels achieved a grade of A* – C. In 2018, 97.4% of its entrants achieved five or more GCSEs at grades A* to C, including the core subjects English and Maths and the college was ranked 28th out of 191 schools in Northern Ireland. In 2017 the college took first place in the Northern Ireland Schools' Analyst Competition which is organised by the local Analytical Division of the Royal Society of Chemistry and involved 12 schools from right across Northern Ireland.
Debating
Thornhill College has a Senior Debating Society, established in 1954. The team were runners-up in the final of the Northern Ireland Schools Debating Competition in 2007, and is known throughout the school for wearing the gold stripe around the lapel of the debating blazer. Thornhill had two teams in the semi-finals of this competition who debated against each other on 30 March 2007 in Belfast. In 2017 the society had two teams in the final of the Queen's Literific Society Debating Competition one of which was the outright winner. The society meets regularly to debate topical subjects and research up and coming competitions. The school also has an established Junior Debating Society who meet after school as part of the homework club. A member from the senior debating society made the national debating team and competed in the 2018 and 2019 World Schools Competition.
Statue
On the "teardrop" at the front of the school there is a piece of sculpture called 'Stepping Out'. The piece was designed by the school's Head of Art and Design, Mr Eamon McAteer and constructed by Maurice Harron and the final sculpture was officially unveiled in 2008. It comprises three abstract figures which represent pupils of the school dancing.
