Recruited out of Canton McKinley High School by Greg Debeljak, McDaniels attended John Carroll University,[1] where he played football mostly as a wide receiver from 1995 to 1998.

His other teammates at John Carroll included London Fletcher, formerly a linebacker with the St. Louis Rams, Buffalo Bills, and Washington Redskins as well as Brian Polian, the former head coach at the University of Nevada-Reno, Jerry Schuplinski, the New York Giants quarterbacks coach, Tom Telesco, former general manager of the Las Vegas Raiders and Dave Ziegler, Telesco's predecessor as general manager of the Raiders.

Throughout the 2008 season, the two met to discuss the report and allow McDaniels to ask non-coaching questions that he brought to later head coaching interviews.

McDaniels directed the Matt Cassel-led Patriots' offense as the team finished the season with an 11–5 record but missed out on the playoffs.

[12] The Broncos introduced McDaniels, who agreed to sign a four-year, $8 million contract, as their head coach in a press conference the next day.

On March 9, 2009, according to ESPN, a conference call involving McDaniels, team owner Pat Bowlen and Cutler failed to resolve the issues.

In the last game of the season, McDaniels and the Broncos still had a potential playoff berth on the line, but lost to the Kansas City Chiefs, 44–24, Denver's third straight home loss to a division opponent.

Scarnecchia told NFL investigators he acted alone and "knew it was wrong" to tape the walkthrough practice, after the rest of the Broncos' staff had left the stadium.

[18] However, the NFL fined McDaniels due to the fact that he did not immediately report the incident to the league office, as required by policy.

"[17] McDaniels later issued the following statement:[19] "I apologize for not promptly reporting the improper conduct of our video director before our game against the 49ers in London.

The actions of this individual are in no way representative of the values and integrity held by myself, our players and coaches, and the entire Denver Broncos organization.

O'Brien left the Patriots after the 2011 season ended to become head coach at Penn State, but maintained play calling duties through Super Bowl XLVI.

During the 2014 season, McDaniels was a part of another championship for the Patriots, winning Super Bowl XLIX against the Seattle Seahawks despite a 10-point deficit in the fourth quarter.

[22] Two days after Super Bowl LII where the Patriots lost 41–33 to the Philadelphia Eagles, on February 6, 2018, McDaniels was announced as the new head coach of the Indianapolis Colts.

[29] He received criticism for Las Vegas's Week 10 loss to an Indianapolis Colts team led by the debuting interim head coach Jeff Saturday, who was hired earlier that week to replace the recently dismissed Frank Reich despite having no previous NFL coaching experience heading into the game.

[34] Before the 2023 season, Jimmy Garoppolo was signed as the Raiders starting quarterback on a three-year, $72.5 million contract, based on his past work with McDaniels when they were both with the New England Patriots.

However Garoppolo was still recovering from an injured foot, so the deal was contingent upon passing a physical prior to Week 1 (which he did) as otherwise the Raiders could have released him without owing compensation.