Mr. Mulgrew assumed his position as Area IV Associate Superintendent in August 2005. He began his teaching career in 1985 as a social studies teacher at Woodbridge Senior High School. He became principal at Brentsville District High School in 1999. He had previously been an assistant principal at Gar-Field High School, his alma mater, from 1995–99.
Under his leadership, Brentsville District High School enjoyed many successes. In 2003, BDHS was awarded the prestigious International Fellowship from the University of Cambridge (U.K.), becoming only the seventeenth centre worldwide to achieve that status. BDHS was also named a model school by the International Center for Leadership in Education in 2005. In 2004, Brentsville was named a NCLB-Blue Ribbon School by the U.S. Department of Education. Brentsville District High School remains the only Virginia high school, either public or private, to ever achieve that status.