[12] After a terrific junior season in which he had 43 tackles, and a Big-Ten high 8 interceptions, King was named a unanimous All-American and won the 2015 Jim Thorpe Award, which is given to the best defensive back in all of college football.

Special teams coach George Stewart named King the secondary kick returner, along with wide receiver Isaiah Burse.

Starting cornerback Jason Verrett suffered a torn ACL during the game and was placed on injured reserve for the rest of the season.

In Week 8, King recorded four solo tackles and made his first career sack on Tom Brady during a 21–13 loss at the New England Patriots.

On November 23, 2017, King recorded five solo tackles, a pass break up, and returned his first career interception off Dak Prescott for a 90-yard touchdown during a 28–6 victory at the Dallas Cowboys on Thanksgiving.

[27] King finished his rookie season in 2017 with 76 combined tackles (66 solo), five pass deflections, and an interception in 12 games and four starts.

Head coach Anthony Lynn named King the first-team nickelback and third cornerback on the depth chart to begin the regular season, behind starters Casey Hayward and Trevor Williams.

[31] On November 4, 2018, King made three combined tackles, broke up two pass attempts, and returned an interception for a 42-yard touchdown during a 25–17 win over the Seattle Seahawks in Week 9.

King intercepted a pass by Seahawks' quarterback Russell Wilson, which was intended for wide receiver David Moore, during the fourth quarter.

[32] In Week 13, King collected a season-high ten combined tackles (nine solo) and returned a punt for a 73-yard touchdown as the Chargers defeated the Pittsburgh Steelers 33–30.

[33][34] He finished the season with 67 combined tackles (47 solo), ten pass deflections, three interceptions, a forced fumble, and one touchdown in 16 games and eight starts.

[36][37] On January 4, 2019, King was named to the Associated Press All-Pro Team, earning first-team accolades as a defensive back and second-team as a punt returner.

On January 6, 2019, King started his first career playoff game and recorded four combined tackles and one sack during the Chargers' 23–17 victory at the Baltimore Ravens in the AFC Wild Card Round.

During the game, King recovered a fumble forced by teammate Jeffery Simmons on David Montgomery and returned it for a 63-yard touchdown during the 24–17 win.