The property on which the school stands was given to John Talbot in 1732 as a land grant from Lord Baltimore. For almost 175 years individual families lived on this property and farmed the land. In 1906, the Mt. Calvary Episcopal School for Boys in Baltimore City bought the property and relocated the school changing its name to the Donaldson School for Boys. As a result of financial losses caused by the Great Depression, the school was forced to close in 1933. The Sisters of Notre Dame de Namur purchased 180 acres of property and the original buildings in 1934 for forty thousand dollars and authorized the opening of a junior high and high school for girls. In 1941, the Sisters opened the elementary school known as the Julie Billiart Country Day School named after the foundress of the Sisters. These students were housed in St. Margaret’s Hall and in the present cafeteria, two of the original cottages of the Donaldson School. In 1958, the name of the school was changed to Trinity Lower School.
Four additions were successfully made to the original cottages of the Donaldson School to complete the Primary School building as it stands today. In June 1972, the girls’ high school was closed and the high school building was leased to the elementary school. This building was designated as the Middle School. When the high school called Trinity Preparatory School closed, the primary and middle schools became officially known as Trinity School and were subsequently incorporated. Since 1972, the school population increased from 245 to the present day enrollment of 380.
In the spring of 1996, Trinity formulated a strategic plan for the future development of the school. This plan called for the construction of a new middle school, improved and expanded roads, sidewalks, parking areas, and athletic fields. Included in the plan was the renovation of St. Lidwine’s Hall, the former middle school building, to be used for administrative offices, art and physical education.
A milestone for Trinity was reached in the spring of 2001, when the Trinity School Board of Trustees purchased the school and the property from the Sisters of Notre Dame de Namur. An Affiliation Plan is attached to the Purchase agreement, ensuring the preservation of the mission, philosophy and long-standing relationship between Trinity and the Sisters of Notre Dame de Namur.
In September of 2002, the new middle school building was officially opened and was dedicated and named St. Julie Hall in November of that year.
The latest major upgrade took place in 2004 when the Kindergarten Cottage was rebuilt to house 2 classrooms for full day kindergarten. In addition, the entire building was renovated and named St. Joseph Hall.
In September of 2009 Trinity opened its doors to 3 and 4 year olds for Pre-School and Pre-Kindergarten.