Koo was born in Seoul, to Seungmae Choi and Hyunseo Koo. He played soccergrowing up, and won a regional kicking competition. He attended sixth grade in the United States after moving to New Jersey to live with his mother, who had gone tothe country two years earlier to become a nurse while Koo's father remained in Seoul as a university professor. Koo played on the football team for Ridgewood High School in Ridgewood, New Jersey. He was named team MVP his senior season, contributing on both special teams and defense, where he tallied six interceptions.
College career
Koo was a four-year contributor for the Georgia Southern Eagles, earning first-team all-conference honors in the Sun Belt in his final season, during which he converted 19 of 20 field goal attempts. Koo was also named a finalist for the Lou Groza Award, given annually to the best college kicker in the nation. At the conclusion of his college career, Koo ultimately converted 88.6% of his field goal attempts, a Georgia Southern team record.
Professional career
Los Angeles Chargers
Koo signed with the Los Angeles Chargers as an undrafted free agent following the 2017 NFL Draft. He won the starting job over incumbent kicker Josh Lambo following the conclusion of the preseason. Koo became the fourth player in NFL history to be born in South Korea. In the 2017 season opener at Denver, Koo's game-tying 44-yard field goal attempt in the final seconds of the game was blocked by the Broncos' Shelby Harris, and the Chargers lost 24–21. The kick had little chance of succeeding after Denver overwhelmed the Chargers' line on the play. An earlier attempt by Koo was successful, but it was called off after the Broncos had called a timeout right before the snap. The following week, Koo missed a game-winning 44-yard attempt in a 19–17 loss to Miami. The following two weeks, Koo converted both of his field goal attempts and all four of his extra points. However, the 0–4 Chargers waived him after Week 4, replacing him with 10-year veteran Nick Novak. Chargers head coach Anthony Lynn stated that he wanted "someone with a little more consistency and experience" than Koo. Lynn added that "I think Koo is going to have a long NFL career... A lot of rookies get cut early in their career, and they come back and play for a lot of years." "Koo's a very talented kicker," Novak said. "He's got a live leg and a lot of talent." Overall with the Chargers, Koo was 3-of-6 on field goals and recorded 8 touchbacks on 14 kickoffs. Frank Schwab of Yahoo Sports wrote that the winless Chargers were "using as a scapegoat."
Atlanta Legends
On January 14, 2019, Koo signed with the Atlanta Legends of the AAF. On February 9, he scored the first points in AAF regular season history, making a 38-yard field goal against the Orlando Apollos. 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. The following week against the Memphis Express, he made all three of his attempts, including the game-winning 35-yard field goal with nine seconds remaining to secure a 23–20 victory, and was eventually named AAF Special Teams Player of the Week. The league ceased operations during midseason in April 2019. For the year, Koo had been a perfect 14-of-14 on his field goals.
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. After longtime kicker Stephen Gostkowski was placed on injured reserve, the New England Patriots signed Koo to their practice squad on October 4, 2019. On October 15, 2019, Koo was released.
Atlanta Falcons
On October 29, 2019, Koo was signed by the Atlanta Falcons after they cut their longtime kicker and franchise leading scorer, Matt Bryant. In his Falcons debut, Koo made all four of his field goals and both of his extra points in a 26–9 upset win over the New Orleans Saints. He was named the NFC Special Teams Player of the Week for his performance. 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. In Week 14 against the Carolina Panthers, Koo made 4 field goals, 4 extra points, and recovered a fumble forced by teammate Damontae Kazee on wide receiverGreg Dortch during a kickoff return in the 40–20 win. For this performance, he was named the NFC Special Teams Player of the Week for the second time. On February 18, 2020, Koo signed a one-year contract extension with the Falcons.
