John G. Costello

John G. Costello grew up in Montgomery County and graduated from Gonzaga College High School. He is a graduate of the University of Delaware and received his J.D. From American University, Washington College of Law. 

John became interested in the legal field at a young age due to his father and grandfather both being attorneys. 

After law school, John served as a law clerk to the Honorable Terrence J. McGann of the Montgomery County Circuit Court. He then worked as an Assistant State’s Attorney in Montgomery County for five years, where he regularly tried criminal cases before judges and juries. In 2016, John entered private practice handling serious personal injury matters on behalf of victims of negligence and representing individuals charged with criminal offenses. 

John is an active member of multiple bar associations and has served multiple terms on the Board of Directors for the Prince George’s County Bar Association. In 2025, John was selected as a Maryland Bar Foundation Fellow. Fellowship in the Maryland Bar Foundation is by invitation only and is restricted to outstanding Maryland attorneys and judges not exceeding two and one-half percent (2.5%) of membership of the Maryland State Bar Association.

Education

• American University Washington College of Law, J.D. – May, 2010
• University of Delaware, B.A. In Communications – May, 2004

Bar Admissions

• Maryland
• District of Columbia
• U.S. District Court, District of Maryland
• U.S. District Court, District of Columbia

Memberships

• Prince George’s County Bar Association
• Montgomery County Bar Association
• Maryland Bar Foundation, Fellow 2025
• Maryland Association for Justice
• National Trial Lawyers Top 100, Member 2022-2024