Bradley joined the North Dakota State Bison as a graduate assistant coach in 1990 and was there until 1991. He was the defensive coordinator/linebackers coach at Fort Lewis College from 1992 to 1995 and spent four months as the team's head coach from December 1995 to March 1996. From 1996 to 2005 Bradley returned to North Dakota State and coached in many positions including defensive coordinator, linebackers coach, and assistant head coach to Bob Babich and Craig Bohl. Under Bradley's guidance, North Dakota State's defense led the Great West Football Conference in scoring defense, pass defense, total defense and turnover margin in 2005.
In 2006, Bradley joined the Tampa Bay Buccaneers as their defensive quality control coach, working closely with defensive coordinator Monte Kiffin in the organization and implementation of the defensive game plan. In 2007 Bradley was promoted to linebackers coach after previous linebackers coach Joe Barry left to become defensive coordinator for the Detroit Lions. Under Bradley's guidance, linebacker Barrett Ruud went on to earn NFC Defensive Player of the Month in September after leading the Buccaneers defense with 51 tackles and adding one interception, three forced fumbles, two fumble recoveries and two passes defensed.
Seattle Seahawks
On January 12, 2009, the Seattle Seahawks hired Bradley to become the team's defensive coordinator. He was recommended for the job by former Buccaneers defensive coordinator Monte Kiffin. In 2009, under Bradley's guidance the Seahawks' defense allowed opponents to gain 356.4 yards and to score 24.4 points per game. The Seahawks finished the season with a 5–11 record, and subsequently replaced head coach Jim L. Mora with Pete Carroll. Carroll decided to keep Bradley for the 2010 season; the two coaches have a common connection with Monte Kiffin. In 2010, the Seahawks' defense allowed opponents to gain 368.6 yards and to score 25.4 points per game. In 2011, the Seahawks' defense allowed opponents to gain 332.2 yards and to score 19.7 points per game. In 2012, the Seahawks' defense allowed opponents to gain 306.2 yards per game and to score 15.3 points per game.
Jacksonville Jaguars
On January 17, 2013, Bradley was hired as the new head coach of the Jacksonville Jaguars. His first regular season win came on November 10, 2013 with a 29–27 victory over the Tennessee Titans. The Jaguars ended the season 4–12, and Bradley was eventually named the head coach of the South team in the 2014 Senior Bowl. On December 18, 2016, the Jaguars relieved Bradley of his duties as head coach, compiling a 14–48 record in 4 seasons.
On January 20, 2017, the Los Angeles Chargers hired Bradley as defensive coordinator under head coach Anthony Lynn. After a 2017 season that saw the defense allow 17 points per game, the third-fewest in the NFL, Bradley received a three-year contract extension in January 2018.
