Callahan attended Cypress Woods High School where he played football. Callahan was named to the 5A All State First-team and Dave Campbell's Texas Football's 2009 All-Texas High School team.
Professional career
Chicago Bears
2015
On May 2, 2015, Callahan signed with the Chicago Bears as an undrafted free agent following the conclusion of the 2015 NFL Draft. He made his NFL debut on September 13 against the Green Bay Packers. On September 22, the Bears waived Callahan. On September 24, the Bears re-signed Callahan for the practice squad. On October 20, he was promoted to the active roster. On November 9, Callahan recorded his first career pass deflection against the San Diego Chargers. On January 3, 2016, Callahan recorded his first career sack against the Detroit Lions. He finished his rookie season with three starts in nine games, notching 21 tackles, four pass breakups, and a sack. He also fielded a punt return for 34 yards.
2016
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 endBrent Celek. On October 9, he made a career-high six tackles but left early due to a hamstring injury against the Indianapolis Colts. On December 11, Callahan broke up a career high two passes against the Detroit Lions. He finished his second season with ten starts in eleven games, recording 32 tackles and five pass breakups. He returned two kicks and one punt, gaining a combined 20 yards.
2017
On March 6, 2017, Callahan re-signed with the Bears. On September 24, he grabbed his second career sack and his first career forced fumble, bringing down Pittsburgh Steelers' Ben Roethlisberger and knocking the football free. On October 15, Callahan grabbed his first career interception, picking off Baltimore Ravens' Joe Flacco and returning it 52 yards. He grabbed his second interception off DeShone Kizer of the Cleveland Browns on December 24. On December 31, Callahan returned a punt on a trick play 59 yards for a touchdown in the season finale against the Minnesota Vikings. His third season ended with six starts in twelve games, recording 25 tackles, six pass deflections, two interceptions, and a sack.
2018
On March 12, 2018, the Bears placed a tender on Callahan with the right of first refusal. On April 17, he signed a one-year deal worth $1.3 million. On September 23, Callahan intercepted quarterback Josh Rosen to seal a 16–14 victory against the Arizona Cardinals. 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. In Week 10 against the Detroit Lions, Callahan made 5 tackles and sacked and intercepted quarterback Matthew Stafford in the 34–22 win. He suffered a broken foot in Week 14 and was placed on injured reserve on December 12, 2018. Callahan finished the season with 45 tackles, 6 passes defended, and 2 interceptions. He received an overall grade of 81.4 from Pro Football Focus in 2018, which ranked as the 7th highest grade among all qualifying cornerbacks.
Denver Broncos
2019
On March 15, 2019, Callahan signed a three-year, $21 million contract with the Denver Broncos, reuniting him with former Bears defensive coordinator and Broncos head coach Vic Fangio. Callahan began training camp as the Denver Broncos’ starting right cornerback opposite 4x Pro Bowl left cornerback Chris Harris Jr. Callahan was also named the starting slot cornerback in sub-packages. However, he was still recovering from a foot injury sustained at the end of the previous season. At the end of the 2018 season, Callahan underwent surgery to repair his broken foot. However, his foot never fully healed and suffered complications from the screw placed in his foot. He was placed on injured reserve on November 15, 2019, without playing a snap in 2019.
