He received his Bachelor of Arts from Furman University cum laude in 2000 and his Juris Doctor from the William & Mary School of Law in 2004, where he was inducted into the Order of the Coif.

He was a law clerk to Judge Robert E. Payne of the United States District Court for the Eastern District of Virginia from 2004 to 2005 and for Judge Roger Gregory of the United States Court of Appeals for the Fourth Circuit from 2005 to 2006.

Before becoming U.S. Attorney, he was a Principal/Partner at Woods Rogers PLC in Roanoke, Virginia, from 2013 to 2018, representing clients in complex civil and criminal litigation.

[1][2] Cullen was recommended as a candidate for United States attorney by Virginia Senators Mark Warner and Tim Kaine.

President Trump nominated Cullen to the seat vacated by Judge Glen E. Conrad, who assumed senior status on December 11, 2017.