The Most Reverend Mark Brennan was born on February 6, 1947 in Boston, MA. to his parents, Edward Charles Brennan and Regina Claire Lonsway (both deceased). He graduated from St. Anthony's High School in Washington D.C. and Brown University in Providence, Rhode Island. He then entered seminary at Christ the King Seminary. For major seminary, he attended the Pontifical NorthAmerican College in Rome, Italy. He was then ordained a priest on May 15, 1976 by Archbishop William W. Baum.
Father Brennan's pastoral assignments include Our Lady of Mercy, Potomac, 1976-1981; St. Pius X, Bowie, 1981-85; St. Bartholomew, Bethesda, 1986-88; St. Thomas Apostle, Washington, D.C., 1998-2003; St. Martin of Tours, Gaithersburg, 2003-2016. He pursued Spanish Language and Cultural Studies in the Dominican Republic and Colombia from 1985-86. From 1988 to 1998, he was Director of Priestly Vocations for the Archdiocese of Washington.