The Sisters of Mercy, who established St Gerard's School, first came to Bangor in 1915. A house - Cooldaragh - was rented by the Sisters of Mercy in Craig-y-Don Road in Upper Bangor, five minutes' walking distance from the school's present location.
Not long after the Sisters' arrival, they opened a school with only two pupils. A steady increase in numbers meant a need for extended premises and a second house - Glyn Menai - was rented a few yards down the road where the girls were placed while the junior boys resided in Cooldaragh.
It soon became evident that these premises were inadequate and a move was indicated. At this point, in 1917, the school's current site became a possibility. Plas Lodwig, the original name for the site, owned by Lord Penrhyn, proved to be ideal and the Sisters negotiated to rent it. They were fortunate also to receive permission to erect a small chapel. Following Lord Penrhyn's death in 1919, the Sisters with the help of the then Bishop of Menevia, Francis Mostyn, purchased the site.
The following years saw further extensions to the building and, in 1960, a general inspection caused the school to be recognised by the Department of Education as 'efficient'. In that year, boarding for junior boys was discontinued but the girls' boarding facility continued until 1972. Thereafter, the school was for girls and junior boys as day pupils until, at the start of the 1980s, boys were admitted to the senior school. The current junior school building was opened in 1975.
The decision of the Sisters to withdraw from independent education in 1990 led to the establishment of a Trust in that year and the appointment, in January 1991, of the school's first lay Head, Anne Parkinson, to succeed the last teaching Sister, Sister Brigid McNally.
In the middle of the following decade, the need to demolish the chapel and to extend the premises led to the erection of the building that currently houses the English and Welsh departments.