Born in the Bronx, Monsignor Robert Brennan was raised in Lindenhurst where he attended Our Lady of Perpetual Help School. He continued his education at St. John the Baptist High School in West Islip, and graduated Summa Cum Laude from St. John's University with a Bachelors degree in Mathematics and Computer Science.

He studied for the priesthood at the Seminary of the Immaculate Conception, Huntington and was ordained in 1989. He served as an associate pastor at St. Patrick Church, Smithtown until his appointment as Secretary to the Bishop in 1994. He worked in that capacity for three bishops: the late Bishops John R. McGann and James T. McHugh, and the present Bishop William Murphy. In 1996, Father Brennan was named an honorary prelate to His Holiness, Pope John Paul II, with the title, "Monsignor." He was named Vicar General in 2002.

In his position as Vicar General, Monsignor Brennan acts in the name of the Bishop as his deputy and assistant with wide ranging responsibilities over the functioning of the Diocese. He also serves as Moderator of the Curia and coordinates the various offices within the Diocese of Rockville Centre.