Originally, this school was located in downtown Guelph at the intersection of Cork and Norfolk Street, next to the Basilica of Our Lady Immaculate. The school's roots date from when the Loreto Sisters, also known as the Institute of the Blessed Virgin Mary, arrived from Toronto and opened a school for girls in 1856. The enrolment quickly increased and in 1872, an addition was constructed. In 1883, it was decided that the younger grades would be moved to two other sites on the original property and the convent school would continue to house the high school students. By 1926, enrolment was continuing to increase and the new Loretto Academy was built immediately next door the convent school. The all-girls school continued to be operated by the Loretto sisters. In 1953, the co-educational Notre Dame High School was erected, and at this time, laywomen and men joined the Sisters as part of the teaching staff. This building provided facilities for grades 9 and 10 and was infamous for its gymnasium and its "caged gallery". On the other hand, Loretto Academy also became co-educational and housed students in grades 11, 12, and 13. In 1962, Loretto Academy and Notre Dame were renamed Bishop Macdonell Catholic High School, after Bishop Alexander Macdonell, a well-respected Catholic leader in his own time, and a few years later in 1966-1967, the two schools were officially joined by an addition. In 1992, a report highlighting the needs of the Catholic community through a Board-School-Church committee, recommended the closure of the original high school and a recommendation of a school of the same name to be built in the future. Despite a long and hard-fought battle with support from many members of the community, the school was closed in June, 1995 and the three buildings, with the exception of the original 1856 convent building, were demolished in 2004. In September 2004, the recommendation of the committee finally came forth as the existing school on Clair Road West opened, starting with grades 9 through 11. Grade 12 has since been added, thereby enabling full granting status of the Ontario Secondary School Diploma, continuing the tradition of the Loretto Sisters and Celtic Pride in Guelph. On June 16, 2009, Grade 9 student Isabel Warren was tragically killed by a cinderblock wall collapsing on her in a washroom facility in Larry Pearson Park, which is located behind the school.
The school now has an approximate population of 750 students ranging from grades 9-12. It is a uniformed school. The uniforms are a blue tartankilt, black dress pants, a golf shirt, and French cut white blouse for girls, and a golf shirt and black slacks for boys. The school's mottos are "Wisdom - Honour - Courage" and "Our Blood Runs Celtic Blue."
