In the first year of operation, there were 47 students, using excess classrooms of the nearby St. John the Evangelist parish.

In 2002, the Diocese of Rockville Centre and the Sisters of Mercy created a partnership to better enhance the needs of the school.

[2] In 2020 The Peconic Bay Medical Center Foundation purchased the 24-acre site of the former McGann-Mercy Diocesan High School from the Diocese of Rockville Centre for $14 million.

The Buildings Usage was for the hospital’s ongoing need for additional space during the Corona Virus Pandemic.

[3] Bishop McGann-Mercy Diocesan High School's theatre department was recognized by the Teeny Awards on an annual basis in recent years, both for performances by student actors and the innovation of the director, Bob Kelly.