2013

The New England Patriots selected Harmon in the third round, (91st overall) of the 2013 NFL Draft. He was the second defensive back selected by the Patriots in 2013, along with Logan Ryan (84th overall).^[8] The selection of Harmon by the Patriots in the third round surprised many draft evaluators and analysts and was seen as a reach. When his selection was announced, ESPN and the NFL Network failed to play his college highlights as typically done with draft announcements since they weren't expecting him to be drafted.^[9][10]

On May 15, 2013, the New England Patriots signed Harmon to a four-year, $2.711 million contract that included a $533,600 signing bonus.^[11]

Throughout training camp, he competed against Adrian Wilson, Steve Gregory, and Tavon Wilson for the job as the starting strong safety after it was left vacant by the departure of Patrick Chung.^[12] Head coach Bill Belichick named Harmon the backup strong safety behind Steve Gregory to start the regular season.^[13]

He made his professional regular season debut in the Patriots' Week 2 matchup against the New York Jets and made one tackle in their 13–10 victory. In Week 8, he deflected a pass and made his first career interception off Ryan Tannehill in their 27–17 victory against the Miami Dolphins. 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. He made three solo tackles in the Patriots' 24–20 loss at the Carolina Panthers. The following week, he made his second consecutive start the following week and recorded a season-high 11 combined tackles during their 34–31 win against the Denver Broncos.^[14] He finished his rookie season in 2013 with 31 combined tackles (24 solo), four pass deflections, and two interceptions in 15 games and three starts.^[15]

2014

Entering training camp in 2014, Harmon was slated to be the starting strong safety after the departure of Steve Gregory.^[16] He competed against Kanorris Davis and Patrick Chung and ultimately lost the job to Chung and was relegated to backup strong safety.^[17]

In Week 6, Harmon recorded a season-high three combined tackles during a 37–22 win at the Buffalo Bills. On December 14, 2014, Harmon returned an interception by Ryan Tannehill for 60-yards as the Patriots defeated the Dolphins 41–13. 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.^[18] "That's the biggest pick of my life," Harmon said after the game. Harmon and the Patriots would go on to win Super Bowl XLIX over the Seattle Seahawks.^[19]

2015–2016

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.^[20] On February 5, 2017, Harmon won his second career Super Bowl championship as the Patriots won Super Bowl LI. In the game, he had three tackles and the Patriots defeated the Atlanta Falcons by a score of 34–28 in overtime. The game featured the largest comeback in Super Bowl history as the Patriots trailed by 25 points in the third quarter.^[21][22]

2017

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.^[23][24] The deal included a $5 million signing bonus and $1.5 million in guaranteed salary.^[25] On September 5, 2017, Harmon was named a Patriots captain for the first time in his career.^[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.^[27] The interception was his fourth of the 2017 season, and his third in the final two minutes of a game.^[28] The Patriots went on to have home field advantage throughout the playoffs and make Super Bowl LII. In the Super Bowl, Harmon recorded five tackles and one interception in a losing effort to the Philadelphia Eagles.^[29]

2018

Harmon switched his jersey number from No. 30 to No. 21 with the arrival of Jason McCourty.^[30] Harmon finished the 2018 season with 38 tackles, four interceptions, and four passes defensed.^[31] He helped the Patriots reach Super Bowl LIII where they beat the Los Angeles Rams 13–3.^[32][33]

2019

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.^[34] He appeared in all 16 regular season games and started eight. He had 22 total tackles (19 solo), two interceptions, and five passes defended.^[35]