McDermott originally joined the Eagles in 1998 as a scouting administrative coordinator, a position he held until being promoted to assistant to the head coach in 1999. He then became defensive quality control coach and later assistant defensive backs coach. In 2003 he replaced Steve Spagnuolo as defensive backs coach, as Spagnuolo was assigned linebackers coach. In 2004, McDermott saw both of his starting safeties (Brian Dawkins and Michael Lewis) earn Pro Bowl berths for the first time in team history. Under McDermott's watch, Dawkins went on to earn two more Pro Bowl berths following the 2005 and 2006 seasons.
In 2007, McDermott was assigned linebackers coach, after Spagnuolo had left for the New York Giants. On January 28, 2008, Eagles head coach Andy Reid named McDermott secondary coach again.
On May 18, 2009, McDermott was named the interim Defensive Coordinator as a result of defensive coordinator Jim Johnson's medical leave of absence. On July 24, 2009, due to the continuing decline of Johnson, the Eagles announced McDermott would take over as full time Defensive Coordinator. Johnson died on July 28, 2009.