On October 14, 1961, groundbreaking ceremonies were held on a 20 acre tract on Martine Avenue in Scotch Plains, New Jersey. The following September, 148 boys and 142 girls were transported by bus from 13 Union County communities to begin their studies in a two-story, partially completed building. The Boys' School and the Girls' School were separate institutions, with their own administration and faculty.

As the school continued to grow in size, so did the academic and extracurricular offerings. In the fall of 1963, 586 new students enrolled and a full athletic program was initiated. With the construction work on the building completed, one side of the building was designated the boys' school and the other wing the girls' school. On June 18, 1966, 127 young women and 90 young men were awarded diplomas as members of the first graduating class of Union Catholic High School.

The years that followed brought about tremendous growth and change. As a result of ongoing evaluation, Union Catholic eventually decided to merge the two schools into one co-educational institution. After the 1979 announcement of the proposed merger, a Search Committee was formed to select a principal for the school. Sister Percylee Hart, RSM, was chosen for the sensitive task of merging the two schools.

The transition that followed required a great amount of cooperation and work from students, faculty, administration, and parents. As each day passed, students became less aware of the separation that had preceded them. Union Catholic was becoming a place where the traditions and customs of all students were blending to form a cohesive community.