He played college football for the Michigan State Spartans, earning unanimous All-American honors and winning the Jim Thorpe Award in 2013.

As a sophomore in 2011, he played in 11 games, missing two due to injury, he finished the season with 42 tackles and three interceptions, two which came against Georgia in the Outback Bowl.

[4][5] Coming out of Michigan State, Dennard was a projected consensus first round pick by NFL draft experts and scouts.

NFL draft analyst Mike Mayock stated that Dennard would have to run a 4.45 or lower in the 40 to be considered a top ten overall pick.

On March 7, 2014, Dennard chose to participate at Michigan State's pro day, along with Max Bullough, Denicos Allen, Bennie Fowler, Dan France, Fou Fonoti, Tyler Hoover, Andrew Maxwell, and nine other teammates.

[15][16][17] Dennard joined a deep position group upon his arrival at training camp and competed with Adam Jones, Terence Newman, Leon Hall, and Dre Kirkpatrick for the job as the starting cornerback.

[18] Head coach Marvin Lewis named Dennard the Bengals' fifth cornerback on their depth chart behind Newman, Hall, Jones, and Kirkpatrick to start the regular season.

The next game, he recorded a season-high three solo tackles and made the first sack of his career on Tennessee Titans quarterback Jake Locker during the Bengals' 33–7 victory.

[20] He saw limited playing time behind the host of a talented veterans during his rookie season and finished with only 17 combined tackles (14 solo), one pass deflection, and one sack in 14 games and zero starts.

On January 4, 2015, Dennard appeared in his first career playoff game and collected two solo tackles and forced a fumble to set up a touchdown during a 26–10 AFC Wildcard loss to the Indianapolis Colts.

[21] Dennard returned to the Bengals' training camp in 2015 and competed with Leon Hall, Kirkpatrick, Jones, and Josh Shaw for the position as one of the starting cornerbacks.

During a Week 4 matchup against the Kansas City Chiefs, he earned a season-high six combined tackles and deflected a pass, as the Bengals won their fourth consecutive game 36–21.

On November 27, 2015, the Bengals officially placed him on injured reserve for the remainder of the season after discovering he had torn ligaments in his right shoulder that would require surgery.

Dennard recovered from his shoulder surgery in time for Bengals' training camp and competed with Jones, Kirkpatrick, Shaw, and rookie William Jackson III.

On September 24, 2017, Dennard recorded a career-high ten combined tackles and sacked Aaron Rodgers during a 27–24 loss to the Green Bay Packers.