On September 19, 2016, in Week 2 against the Philadelphia Eagles on Monday Night Football, Callahan made a diving play to deflect a touchdown pass from quarterback Carson Wentz to tight end Brent Celek.

[13] On October 15, Callahan grabbed his first career interception, picking off Baltimore Ravens' Joe Flacco and returning it 52 yards.

[15] On December 31, Callahan returned a punt on a trick play 59 yards for a touchdown in the season finale against the Minnesota Vikings.

[19] On October 28, Callahan made 4 tackles, deflected 3 passes and sacked quarterback Sam Darnold in a 24–10 victory against the New York Jets.

[20] In Week 10 against the Detroit Lions, Callahan made 5 tackles, sacked, and intercepted quarterback Matthew Stafford in the 34–22 win.

[24] Callahan entered training camp as a starting cornerback opposite A. J. Bouye and rookie Michael Ojemudia.

He finally made his debut with the Broncos as the starting slot cornerback in Week 1 of the 2020 season against the Tennessee Titans on Monday Night Football.

[25] In Week 6 against the New England Patriots, Callahan recorded his first interception as a Bronco off a pass thrown by Cam Newton during the 18–12 win.

At the conclusion of the NFL preseason, head coach Vic Fangio officially retained Callahan as the starting slot cornerback.

[30] Callahan finished the 2021 NFL season appearing in 11 games (6 starts) and collected 29 tackles (25 solo) along with 4 passes defensed, 1 sack, and no interceptions.