Simmons performed at private workouts with Dallas Cowboys and New Orleans Saints and also attended pre-draft visits with the Pittsburgh Steelers and Detroit Lions.

[14] Head coach Gary Kubiak named Simmons the backup free safety to begin the regular season, behind Darian Stewart.

[18] On November 13, 2016, Simmons leaped over the offensive line and blocked an extra point attempt by Saints' kicker Wil Lutz.

[20] He collected a season-high five combined tackles, broke up two passes, and made his first career interception in the Broncos' 33–10 loss at the Kansas City Chiefs in Week 16.

Simmons intercepted a pass by Chiefs' quarterback Alex Smith, that was initially intended for wide receiver Tyreek Hill, and returned it for a 38-yard gain in the first quarter.

[21] Simmons finished his rookie season in 2016 with 30 combined tackles (25 solo), four pass deflections, two interceptions, and one sack in 13 games and three starts.

[23] On January 11, 2017, the Denver Broncos hired Miami Dolphins defensive coordinator Vance Joseph as their new head coach.

Head coach Vance Joseph named Simmons the starting strong safety to begin the regular season and elected to release T. J.

On December 3, 2017, he made seven combined tackles, broke up a pass attempt, and returned an interception for the first touchdown of his career during a 35–9 loss at the Dolphins in Week 13.

Simmons intercepted a pass thrown by Dolphins' quarterback Jay Cutler, that was intended for wide receiver DeVante Parker, and returned it for a 65-yard touchdown during the third quarter.

[27] Simmons finished the season with 68 combined tackles (49 solo), five passes defensed, two interceptions, one sack, and one touchdown in 13 games and 13 starts.

[29] Simmons started all 16 games in 2018 and recorded a career-high 97 combined tackles (71 solo), four pass deflections, and three interceptions while playing every single defensive snap for the first time in his career.

[32] Fangio was a part of former Denver Broncos' head coach John Fox's staff in Chicago, as well as the new defensive coordinator Ed Donatell.

[38] On September 20, 2020, Simmons led the team with eight combined tackles (seven solo) and had a season-high two pass deflections while also making his first interception of the season thrown by Ben Roethlisberger during the 26–21 loss at the Pittsburgh Steelers.

[43] He started in every game for the third consecutive season in 2020 and finished with a total of 96 combined tackles (77 solo), nine pass deflections, and a career-high five interceptions.

On March 5, 2021, the Denver Broncos placed the franchise tag on Simmons for a second consecutive season, offering a one-year, $13.73 million contract.

[47] In Week 12, he collected a season-high eight combined tackles (seven solo) and deflected two passes in the Broncos' 28–13 win over the Los Angeles Chargers.

[48] On January 27, 2022, the Denver Broncos hired Green Bay Packers' offensive coordinator Nathaniel Hackett as their head coach.

In Week 15, he had four combined tackles (two solo) and two interceptions from passes thrown by Colt McCoy and Trace McSorley as the Broncos defeated the Arizona Cardinals 24–15.

On January 1, 2023, he recorded a solo tackle, a pass deflection, and set a career-high with his sixth interception of the season after picking off Patrick Mahomes during a 24–27 loss at the Kansas City Chiefs.

He ended the 2022 NFL season with 69 combined tackles (42 solo), seven pass deflections, six interceptions, and a forced fumble in 12 games with 12 starts.

In Week 8, Simmons played a large role in the Broncos ending their 16-game losing streak to the Kansas City Chiefs, recording two tackles, a fumble recovery, and a fourth-quarter interception in a 24–9 victory.

[60] Although Simmons was signed towards the end of training camp, he competed for a role as a starting safety against Richie Grant and DeMarcco Hellams.

[64] In the summer of 2009, Simmons played in the USSSA U15 Florida state basketball championship game in Gainesville against the Jupiter Jaguars, a team that featured Tyler Cameron, future football player at Wake Forest and Florida Atlantic and member of season 15 of ABC's The Bachelorette as well as Kedric Bostic, future Quarterback at Princeton University, among others.