Secondary Education came to Edenderry in 1939 on the initiative of Sr. Laserian Hasset of the order of St. John of God. In that year a Secondary Top was established to promote opportunities for girls to enter for public examinations. In its first year of existence the Secondary Top had an enrollment of thirty-five pupils and this increased steadily over the years. Girls sat the Leaving Certificate for the first time in 1943 when two girls took that examination.
Secondary education for boys in Edenderry began on 2nd September 1960 when twenty five boys, some from the town, others cycling distances of ten to twelve miles, arrived to a borrowed room at the present Boys’ National School. There they were met by Mrs. Malone and Mrs. Fennessy who in time were to become legendary figures in educational circles throughout the northern end of the Diocese of Kildare and Loughlin. The following year the boys and their three teachers moved to the present St Joseph's Hall. In time they were to occupy all five rooms. For the first time, five boys sat the Leaving Certificate in 1969.
The numbers of boys and girls seeking secondary education increased steadily throughout the 1960's and the 1970's. They had to be accommodated somewhere. The prefab era came into existence and eventually there were twenty three prefab rooms located between where the all-weather pitch beside St. Joseph's Hall and the boiler house of the new secondary school now stand. As numbers increased the curriculum expanded. Boys sought to learn what had been traditionally girls’ subjects and girls sought to be taught what had been traditionally boys’ subjects. Co-education had arrived. The two schools began operating a common time table. The two principals, St Camillus and Mr Eddie Lynch cooperated fully towards the next logical step, an amalgamation of the two school which came about in 1982.
The far seeing Monsignor Brenan P.P., who was Manager of both schools at the time, had over a period of years secured ten and a half acres which were ready for vesting with the Department of Education. St. Mary's Secondary School as we know it now opened its doors to 806 students during the first week of September 1982. In 1984 Mrs Bernie O'Neill was appointed Principal of the amalgamated school and Mr Eddie Lynch became Vice Principal. Along with their dedicated staff they built the school in to a major educational venue with enormous influence on Edenderry and surrounding areas.
An extension was added in 2002 and it included a state of the art gymnasium. A further extension was added to the school in 2011 which houses a fully equipped Woodwork room and 4 class-rooms. Mr Lynch retired from his position in August 2009 to be replaced by Mr Rob Halford. Mr Halford had taught in a number of schools in Dublin and Kildare and brought with him many new ideas and initiatives.
2012 brought huge change to St Mary's. Mrs. O'Neill retired after 28 years of dedicated service to St. Mary's. Her legacy is outstanding. Mr. Halford was appointed Principal to replace Mrs O'Neill. Mrs. Margaret O'Kelly, a teacher in the school for over thirty years was appointed deputy principal. Together they formed a very strong leadership team for the next five years.
Following Mrs O'Kelly's retirement in 2017 the school appointed Mr Emmet McDonnell and Ms Sharon Ryan as deputies principals to form a new, bigger leadership team. The leadership team are joined by a strong senior management team and a range of other post holders. Teacher numbers grew to 62 in 2019, supported by several part time teachers, Special Needs Assistants and dedicated support staff. The school is currently busy embracing the New Junior Cycle, the revised Child Protection Procedures and the Europe wide General Data Protection Policies as well as raft of new policies and new technologies that support teaching and learning.
St Mary's enrolment has now reached 873 (September 2019). An ambitious five year Strategy Plan, set to to be published in Dec 2019, sets out a broad range of targets and actions as the school sets to sustain its place as one of the most successful secondary schools in the Midlands.