[14] Head coach Andy Reid named Thornhill the backup free safety, behind Daniel Sorensen, to begin the regular season.

[15] During pre-game warmups, head coach Andy Reid unexpectedly chose to start Thornhill at free safety immediately after kickoff.

[16] He made his first career start in his professional regular season debut in the Kansas City Chiefs' season-opener at the Jacksonville Jaguars and recorded a season-high eight combined tackles (five solo) during a 40–26 victory.

He finished his rookie campaign with a total of 58 combined tackles (42 solo), five pass deflections, three interceptions, and one touchdown while starting all 16 games.

[21] The Kansas City Chiefs finished the 2019 NFL season first in the NFC West with a 12–4 record, clinching a playoff berth and first round bye.

[26] The Kansas City Chiefs continued improving as they finished the 2020 NFL season with a 14–2 record, to earn a playoff berth and first round bye.

On January 17, 2021, Thornhill appeared in his first career postseason game, but was limited to one tackle and a pass deflection during a 22–17 victory against the Cleveland Browns in the Divisional Round.

Head coach Andy Reid named Thornhill a backup to begin the regular season, behind starting duo Daniel Sorensen and Tyrann Mathieu.

He finished the 2021 NFL season with a total of 64 combined tackles (43 solo), three pass deflections, and one interception while appearing in all 17 games with 12 starts.

Head coach Andy Reid selected Thornhill to start at free safety throughout the playoffs, alongside Tyrann Mathieu.

On January 30, 2022, Thornhill started in the AFC Championship Game and made six combined tackles (two solo) as the Chiefs lost 24–27 to the Cincinnati Bengals.

On January 21, 2023, he made five combined tackles (three solo) and a season-high tying two pass deflections during a 27–20 win against the Jacksonville Jaguars in the Divisional Round.

On February 12, 2023, Thornhill started at free safety in Super Bowl LVII and made five combined tackles (three solo) and a pass deflection as the Chiefs defeated the Philadelphia Eagles 38–35.

Thornhill entered training camp slated as the starting free safety under defensive coordinator Jim Schwartz.

Head coach Andy Reid named Thornhill and Grant Delpit as the starting safeties with Rodney McLeod as the nickelback.