In the 2017 season opener on the road against the Denver Broncos, 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 the Miami Dolphins.

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.

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 (including a 48-yarder) 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 four field goals (including a 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. 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.