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 school was established to promote opportunities for girls to enter for public examinations. In its first year of existence the school had an enrolment 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 at 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 1960s and the 1970s. This growth in numbers led a need for more rooms than were available in St. Joseph’s hall. As a result, the prefab era came into existence and twenty-three prefab rooms were 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 in Edenderry. The two schools began operating a shared timetable. 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 560 students during the first week of September 1982. In 1986 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 into 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 classrooms. Mr Lynch retired from his position in August 2009 to be replaced by Mr Rob Halford. Mr Halford had taught in several 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 26 years of dedicated service to St. Mary’s. She left a proud legacy behind her for others to build upon.  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 deputy principals to form a new, bigger leadership team.  The leadership team is supported by a strong middle management team.  Teacher numbers grew to 62 in 2019, supported by several part time teachers, special needs assistants and dedicated support staff.  The school has embraced 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. 

In 2021, following Rob Halford’s retirement, Emmet McDonnell was appointed principal and put a new senior leadership team in place. Under the guidance of Emmet McDonnell, and the Deputy Principals: Tom Reehill, Brenda Lawton, and David Flynn, St. Mary’s Secondary School continues to provide a well-rounded education that combines intellectual growth with the development of character, leadership skills, and a sense of social responsibility. At St Mary’s, our mission is to focus on nurturing the whole person and to ensure that students receive a well-balanced education, empowering them to reach their full potential, and contribute positively to society. This aim is most highlighted in the continuous expansion of St Mary’s Secondary School. As we embrace this new chapter in the life of St. Mary’s Secondary School, we honour the profound contributions of previous leadership, and express our deep gratitude for their dedication to the school community. We are confident that the school’s cherished legacy will continue to thrive and that future generations of students will be inspired, challenged, and guided by the values that have defined St. Mary’s throughout its history. 

In September St. Mary’s opened its newest extension. This is a fantastic addition to our campus and includes a Science lab, DCG room, new classrooms, and a state-of-the Technology room. These additions will allow our students to develop the skills they need to flourish in the digital world in which we now live. 
 
St Mary’s enrolment has now reached 998 (September 2023).  An ambitious five-year Strategy Plan, 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. 

There have been many changes to the school across more than 80 years. The coming years and decades will see more growth and change. However, no matter what changes the future brings, St. Mary’s will always remain a student-centred school, where every decision is made for the benefit of our students. St. Mary’s will continue to be not only a centre of academic excellence but a place where all students are valued, nurtured, and cared for. Our goal is to support students as they grow and develop, and to help them become the best that they can be.