[1][2][3] Harmon has also played for the Detroit Lions, Atlanta Falcons, Las Vegas Raiders, Chicago Bears and Cleveland Browns.

[5] Following his senior season in 2012, he was a first-team All-Big East Conference selection at safety[6] and participated in the NFLPA Collegiate Bowl.

At the conclusion of the pre-draft process, Harmon was projected to be a seventh round pick or undrafted free agent by NFL draft experts and scouts.

[12] Head coach Bill Belichick named Harmon the backup strong safety behind Steve Gregory to start the regular season.

The following week, he recorded four combined tackles, deflected a pass, and returned an interception by Ben Roethlisberger for 42 yards during their 55–31 victory against the Pittsburgh Steelers.

On November 18, 2013, Harmon earned his first career start in place of Steve Gregory, who suffered an injured finger earlier in the week.

[15] Entering training camp in 2014, Harmon was slated to be the starting strong safety after the departure of Steve Gregory.

In the divisional round of the 2014 playoffs, Harmon intercepted Joe Flacco's pass with less than two minutes remaining in the game allowing the Patriots to hold onto a 35–31 win over the Baltimore Ravens.

[19] In the 2015 and 2016 seasons, Harmon was mainly used as the Patriots' third safety alongside Devin McCourty and Patrick Chung primarily in nickel and dime packages.

In the Divisional Round of the 2016 playoffs, Harmon intercepted Houston Texans' quarterback Brock Osweiler—after fellow Rutgers alums Devin McCourty and Logan Ryan each had picks of their own—in a 34–16 victory.

The game featured the largest comeback in Super Bowl history as the Patriots trailed by 25 points in the third quarter.

[21][22] On March 9, 2017, Harmon, who was a free agent at the time, signed a four-year contract worth a maximum of $20 million to remain with the Patriots.

[26] On December 17, 2017, during Week 15 against the Pittsburgh Steelers, Harmon made the game-sealing interception as the Patriots won 27–24, allowing them to clinch the AFC East.

[32][33] In week 6 against the New York Giants, Harmon recorded his first interception of the season off Daniel Jones and returned it for 27 yards in the 35–14 win.

Harmon was detained and denied entry, being sent back to the United States after marijuana and paraphernalia were found in his luggage.