The Congregation of the Sisters of Mercy was founded in Dublin in 1831 by Catherine McAuley. Catherine McAuley was born in North Dublin in 1778. Following the death of her father in 1783, life for her became difficult both economically and socially. As a teenager she nursed her mother through a prolonged illness until she died. After that, economic circumstances necessitated moving from one relative to another in order to have a roof over her head. Sometimes this meant being separated from her brother and sister.

In 1803, Mr. and Mrs. William Callaghan, a wealthy childless couple, adopted her. She moved with them to live in Coolock in the outskirts of Dublin. While she was living with them she commenced her mission of mercy to the neighbouring poor and sick. In addition she nursed Mrs. Callaghan through a lengthy illness until she died in 1819 and subsequently nursed Mr. Callaghan before his death in 1822. The Callaghan estate was left to Catherine McAuley and she used it to fund her building development in Baggot Street for the care of the poor and socially disadvantaged.

In 1831 she founded the Sisters of Mercy, a Religious Congregation largely involved in the care of the poor, the sick and educationally disadvantaged. In the early days her work was mostly among the people of Dublin, but in time the Congregation spread and became one of the largest Congregations of women, not only in Ireland, but in the world.

Catherine McAuley was a woman of great vision. She was an innovator in what she did, but also in how she did things. The care of the sick was always close to her heart and so in 1832, when an outbreak of cholera in Dublin was claiming hundreds of lives and people fled in fear of succumbing to the disease, she organised a team of Sisters to care for the sick at an improvised hospital in Townsend Street.

Her life as a Sister of Mercy only spanned ten years. In that time she worked tirelessly to respond to the need of the poor and sick. She set up a number of foundations for this purpose both in Ireland and England and was about to make her first foundation in America when illness overtook her. She died in 1841.
 
The Congregation of the Sisters of Mercy spread rapidly in Ireland, England, America and Australia to become one of the largest congregations of women in the church. In Dublin the sisters became known as walking nuns because they were seen about the streets visiting the sick at home and in hospital and engaging in their works of mercy. Today the Sisters are to be found in many parts of the Third World in education and nursing.

At home, they work in parishes, schools and hospitals bringing the mercy of God to those whom they serve by alleviating suffering wherever they find it.

The Sisters arrived in Downpatrick in June 1855 and founded their convent in Irish Street. Within a few weeks they opened a primary school in two rooms in the same building and had an initial enrolment of 109 girls and 26 boys. This school, originally called St Philomena's, became the Convent of Mercy Primary School. Both convent and primary school therefore celebrated their 150th anniversary in 2005. The Convent of Mercy Primary School amalgamated with St Patrick's Boy's Primary School in September 2011.

Mother Aloysius was the first Principal in 1957 and Sr Teresa the Vice Principal.

St Mary's Girls' Intermediate (as it was originally known), was opened on 8 September 1957, by the late Most Rev Dr Mageean, Bishop of Down and Connor, assisted by the late Archdeacon McWilliams, Parish Priest of Downpatrick (see photograph below). There were 286 pupils enrolled and 12 teachers employed.

The first staff established the ethos and discipline which still prevails. The Sisters of Mercy brought to the new school their expertise in the field of commercial education and before long they recognised the need to widen the scope of the courses offered. In 1960, after lengthy negotiations with the then Ministry of Education, the school was given the right to provide a 'grammar stream' - one of the first secondary schools to achieve this status. This enabled pupils to take junior, senior and advanced certificates and pursue careers which would otherwise have remained inaccessible.
 
In the 1970s under the dynamic leadership of Sr Teresa the school's standing continued to rise. This was exemplified in a dramatic increase in school numbers to over eight hundred pupils and in continued academic success in public examinations.

As the curriculum broadened music, drama and sport were given a higher profile. The decade heralded a period of success in the creative arts. The 'Musical' became an annual event and many past pupils will have fond memories of 'Blossom Time' and 'My Fair Lady'. A high level of success was always achieved in Speech and Music sections of the Belfast and other festivals.

The 1980s brought a period of change in education with an emphasis on science and technology. It fell to Sister Anne as Principal to implement these changes which were somewhat 'novel' in a girls' school at that time. These she managed with thought and sensitivity, using them as a prelude to the changes she would face with the advent of the National Curriculum. Conservation and environmental topics were always at the head of her agenda. She encouraged the pupils to be aware of third world issues, in particular the plight of the street children of Brazil.

At this point it would be appropriate to pay tribute to Sister Mercy, who tirelessly served the school for 28 years. She will be remembered for her dedication to the pupils, her meticulous attention to detail and her wealth of knowledge. Former staff appreciated her supportive role in the day to day running of the school and many past pupils and teachers will recall her phenomenal memory.

Miss Murphy guided St Mary's through the 1990s, serving as Principal from September 1992 to August 1998. A past pupil of St Mary's she returned to join the teaching staff in 1973 and became Vice Principal in 1985. Her time as Principal was a period of great change in St Mary's, which saw the Official Opening of the 'New Extension' in 1993 (see photograph left) and continued curriculum changes heralded the arrival of ICT and Technology. In 1992 the sixth form was re-established with a range of 'A' Level and Post 16 courses offered. GNVQ courses began in 1994 and this formed the foundation of our present Post 16 Curriculum.

Mrs Smyth was appointed in 1998 and led the school through another period of challenge and change during which the school re-introduced a choice of 'A' Level subjects to supplement the AVCE and GNVQ courses already on offer. The curriculum has been broadened at Key Stage 3 and Key Stage 4 to include Drama and Media Studies. A planned programme of improvement of the school environment has taken place alongside the provision of ICT resources to accommodate the demands for courses in this area.
 
The school, by now, was embarking on links with other schools in the European Community by e-mail with teacher and pupil exchanges to follow. All this hopefully prepared staff and pupils to meet the challenges of the future ahead.

Mrs Darling was appointed as Principal from 2010-2019.

Ms Mc Laughlin commenced her role as Principal of St. Mary's High School in March 2019 to the present day. The school is constantly changing to meet the needs of every child within its community.