In his first season, he led the Bills to their first playoff appearance since 1999, ending the longest active postseason drought in the four major North American sports.
He was teammates with future Pittsburgh Steelers head coach Mike Tomlin, who was a wide receiver and faced McDermott frequently.
[citation needed] McDermott originally joined the Philadelphia Eagles in 1999 as a scouting administrative coordinator, a position he held until being promoted to Andy Reid's coaching staff in 2001.
[4] 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.
[6] In 2007, McDermott was assigned linebackers coach, after Spagnuolo had left to take the defensive coordinator job for the New York Giants.
On July 24, of that same year, due to the continuing decline of Johnson, the Eagles announced McDermott would take over as full-time defensive coordinator.
However, they lost the next two games, including a 47–10 loss to the New Orleans Saints, which prompted him to make the controversial decision to bench starting quarterback Tyrod Taylor in favor of rookie backup Nathan Peterman.
Peterman played poorly against the Los Angeles Chargers in his first career start, throwing 5 interceptions in the first half.
[27] Despite the string of losses, the Bills then went on a 4–2 run to finish the season at 9–7, clinching the AFC's 6th seed and their first playoff appearance in 18 years, thus ending both the NFL's and the North American professional sports franchise's longest active playoff droughts during McDermott's first year as head coach.
Quarterback Josh Allen developed dramatically, turning into an MVP candidate and leading the Bills to their first AFC East Division Title since 1995, as well as a 13–3 record, tied for second best in the league with the Green Bay Packers and behind the Kansas City Chiefs.
[37] Prior to the 2021 season, the Bills increased protection for Allen by drafting offensive tackle Spencer Brown from Northern Iowa.
The Bills had a record of 5–1 within the division, with the sole loss to New England having taken place during particularly poor weather—wind gusts nearby were measured at over 55 miles per hour.
However, Buffalo's defense would not hold, and Kansas City was able to score a field goal to tie the game 36–36 as time expired.
The Chiefs then won the coin toss for possession to begin overtime, and ultimately scored a touchdown on the resulting drive to win the game 42–36.
[46] After the season, partially in reaction to this result—given Buffalo's offense was not given a chance to participate in the overtime period—the NFL changed their postseason rules to guarantee both teams a possession of the ball, even if the opening drive results in a touchdown.
[48] The Bills did not play 17 games in 2022 due to the Damar Hamlin incident in Week 17, which McDermott got praise for helping cancel.
[63] Players reportedly were unsure how to answer McDermott, with one jokingly saying TSA was the biggest difficulty to their plan when he was asked a question about it.