Rapp was born in Atlanta, Georgia, to a Chinese mother from Shanghai and a Caucasian father who was raised in Canada. After his birth, his parents decided to move to the West Coast. He and his older brother by three years, Austin, were raised primarily in Bellingham, Washington, where they both attended Sehome High School. NFL.com described Rapp as a player who has "beaten long odds, undeterred by a city that failed to support prep football, a high school community that openly mocked him, coaches who were unprepared to develop his talents, college recruiters who blatantly overlooked him and kids who racially taunted him for his Chinese ethnicity." Rapp played safety and running back in football and also ran track. In high school, Rapp played football for the U-16 and U-17 USA national teams. He committed to Washington in the spring of his junior year, choosing the Huskies over several FBS programs including Oregon, Stanford, Nebraska, and Notre Dame. Rapp was ranked as the #1 safety prospect in the state of Washington and the 9th best safety on the West Coast by Scout.com. He received an invitation to play in the prestigious U.S. Army All-American Bowl game, but elected not to attend as it interfered with his schedule after enrolling early at the University of Washington.
College career
Freshman
Rapp enrolled early at UW in January 2016 and participated in winter workouts and spring practices. He saw playing time in all 14 games of his freshman season, starting 10 of them. He started to emerge as a play maker in the secondary late in the season after nabbing a pair of interceptions against USC. In the 2016 Pac-12 Football Championship Game, Rapp intercepted back-to-back passes from Colorado QB Sefo Liufau, returning one of them for a touchdown. For his outstanding performance, he was named the Pac-12 Championship Game MVP. Following the regular season, Rapp was named to the USA TodayFreshman All-America team. Rapp was recognized by the Pac-12 as the conference's Freshman Defensive Player of the Year. On December 12, Rapp was named to the ESPN True Freshman All-America team. On December 19, he was named to the Pro Football Focus Freshman All-American team. On January 9, Rapp was named to the Football Writers Association of America Freshman All-America team.
Sophomore
Prior to the 2017 season, Rapp was named one of the 100 best players in college football by Sports Illustrated, coming in at number 39 on the list. Earning first-team All-Pac-12 honors for the 2017 season, he was also named to the Academic All-Pac-12 first team. He was also named a CoSIDA Academic All-District 8 player. Rapp gained acceptance into the University of Washington's prestigious Foster School of Business late in 2017.
Junior
Prior to his junior season, Rapp was named a first-team All-American by the Associated Press and Sports Illustrated. On January 2, 2019, Rapp announced that he would forgo his final year of eligibility and declare for the 2019 NFL Draft.
Professional career
The Los Angeles Rams selected Rapp in the second round of the 2019 NFL Draft. He was the fifth safety drafted in 2019.
2019
On June 7, 2019, the Los Angeles Rams signed Rapp to a four-year, $4.67 million contract that includes $2.12 million guaranteed and a signing bonus of $1.41 million. In week 13 against the Arizona Cardinals, Rapp recorded an interception off fellow rookie Kyler Murray and returned it for a 31 yard touchdown in the 34–7 win. This was Rapp's first career interception and touchdown in the NFL. In week 16 against the San Francisco 49ers on Saturday Night Football, Rapp recorded 5 tackles during the 34–31 loss. With 58 seconds left in the game on third and 16, Rapp and cornerback Jalen Ramsey made an error in coverage which resulted in a 46 yard completion to wide receiverEmmanuel Sanders, eventually leading to a game winning field goal which knocked the Rams out of playoff contention. In week 17 against the Arizona Cardinals, Rapp recovered a fumble lost by Kyler Murray and intercepted another pass thrown by Murray during the 31–24 win.
