On May 8, 2015, the New England Patriots signed King as an undrafted free agent after he went unselected in the 2015 NFL Draft. The contract King agreed to was for three-years and $1.57 million. With the Patriots depth at both safety positions, King was relegated to competing for a role on special teams. On September 5, 2015, King was released by the Patriots as part of their final roster cuts and was signed to the practice squad the next day. On October 10, 2015, the Patriots signed King to their active roster after releasing cornerback Bradley Fletcher. The following day, he made his professional regular season debut and recorded two solo tackles in a 30-6 victory at the Dallas Cowboys. On November 23, 2015, King collected two solo tackles and forced the first fumble of his career in a 20-13 victory over the Buffalo Bills. During the third quarter, King was assigned to be the gunner on the punt recovery team, opposite Matthew Slater. He forced the Buffalo Bills' punt returnerLeodis McKelvin to fumble and it was recovered by Patriots' linebacker Jonathan Freeny. King finished his rookie season with 12 combined tackles in 13 games and played solely on special teams. His 12 special teams tackles finished second on the team behind only Matthew Slater's 17. The New England Patriots finished the season atop the AFC East with a 12-4 record. On January 16, 2016, King played in his first career playoff game and assisted on a tackle during the Patriots' 27-20 victory In the AFC divisional game against the Kansas City Chiefs. The following game, he recorded two solo tackles in the Patriots 20-18 loss to the eventual Super Bowl 50 Champions the Denver Broncos.
2016
On April 7, 2016, King signed his exclusive rights free agent tender to remain with the Patriots. The contract terms were for one-year and $525,000 King competed with Cedric Thompson throughout training camp for a job as a backup strong safety. He was named the third string strong safety behind Patrick Chung and Jordan Richards to start the season. On October 16, 2016, King recorded a season-high two combined tackles during a 35-17 victory over the Cincinnati Bengals. He finished the season with ten combined tackles in 16 games and zero starts. On February 5, 2017, King was part of the Patriots team that won Super Bowl LI. In the game, the Patriots defeated the Atlanta Falcons by a score of 34–28 in overtime. Throughout the playoffs, he appeared in all three games and assisted on one tackle in the AFC divisional round 34-16 victory over the Houston Texans.
2017
On April 12, 2017, King signed his exclusive rights free agent tender to remain with the Patriots. He made the Patriots' 53-man roster after establishing himself as a core special teamer in his previous two years with the team. On October 29, 2017, King forced his first career safety while defending a punt return when he tackled Travis Benjamin in the endzone during the Patriots' 21-13 victory over the Los Angeles Chargers. The Patriots would reach Super Bowl LII, but lost 41-33 to the Philadelphia Eagles with King recording 1 tackle.
2018
On March 8, 2018, King signed a two-year contract extension with the Patriots for $2.6 million. King helped the Patriots reach Super Bowl LIII, which they ended up winning against the Los Angeles Rams by a score of 13-3 on February 3, 2019.
2019
On May 17, 2019, King signed a two-year contract extension with the Patriots through the 2021 season. He was placed on injured reserve on August 25, 2019 with a torn quad.