[4] Koo played four seasons of college football for the Georgia Southern Eagles and was named a finalist for the Lou Groza Award in 2016.

He signed with the Los Angeles Chargers as an undrafted free agent in 2017 but was released a month into the season after missing consecutive game-ending kicks.

[9][10] At Georgia Southern University, Koo was a four-year contributor for the Eagles, earning first-team all-conference honors in the Sun Belt Conference in his final season, during which he converted 19 of 20 field goal attempts.

[11] At the conclusion of his college career, Koo ultimately converted 88.6% of his field goal attempts, a Georgia Southern team record.

[25] After the Legends started the 2019 AAF season 0–3, on March 3, Koo kicked two field goals, including the 33-yard game winner, against the Arizona Hotshots to seal the 14–11 Atlanta win.

[21] Following the suspension of the AAF, Koo worked out with the Chicago Bears, who were in need of a kicker after releasing Cody Parkey, but did not end up signing with the team.

[29] After longtime kicker Stephen Gostkowski was placed on injured reserve, the New England Patriots signed Koo to their practice squad on October 4, 2019.

[31] On October 29, 2019, Koo was signed by the Atlanta Falcons after they cut their longtime kicker and franchise leading scorer, Matt Bryant.

[32] In his Falcons debut, Koo made all four of his field goals (including a 48-yarder) and both of his extra points in a 26–9 upset win over the New Orleans Saints.

[34] In Week 13 against the New Orleans Saints on Thanksgiving Day, Koo delivered three successful onside kicks, one of which was undone by a penalty, in the 26–18 loss.

[35][36] In Week 14 against the Carolina Panthers, Koo made four field goals (including a then career-long 50-yarder), four extra points, and recovered a fumble forced by teammate Damontae Kazee on wide receiver Greg Dortch during a kickoff return in the 40–20 win.

[49] In Week 17 against the Arizona Cardinals, Koo made another game-winning field goal to give the Falcons a 20–19 win as time expired.

[51] In Week 2 against the Green Bay Packers, Koo kicked a go-ahead game-winning field goal with less than a minute left to give the Falcons a 25–24 win.

[57] The following week against the Tampa Bay Buccaneers, he hit a game-tying 52-yard field goal to end regulation as the Falcons went on to win in overtime 36–30.