Kieboom attended George Walton Comprehensive High School in Marietta, Georgia. He committed to attend Clemson University to play college baseball for the Clemson Tigers. He was drafted by the Washington Nationals in the first round of the 2016 Major League Baseball Draft. He announced he would be joining the organization on June 11, 2016. He spent his first professional season with the GCL Nationals where he batted.244 with four home runs and 25 RBIs. In 2017 with the Class-A Hagerstown Suns, he was hitting.333 and six home runs and 20 RBIs before a hamstring injury on May 12 placed him on the disabled list. Kieboom was named to the Northern Division All-Star team in the South Atlantic League, alongside several Suns teammates. Both Baseball America and MLB Pipeline listed Kieboom as the Nationals' fourth-best prospect as of their 2017 season. After returning to action late in the season, Kieboom played in six games with the GCL Nationals and seven games with the Auburn Doubledays before returning to Hagerstown. Across all three levels, he had a.297 batting average with nine home runs in 61 games for the 2017 season. Playing for the High-A Potomac Nationals in 2018, Kieboom was named a Carolina League All-Star, one of six Potomac players so honored. In the All-Star Game, he hit a home run while going 3-for-5. He was promoted to the Class-AA Harrisburg Senatorsfor the first time following the All-Star Game, homering off Baltimore Orioles prospect Keegan Akin in his first game at the higher level on June 21, 2018. Representing Team USA, he was one of two Nationals minor league players, along with Dominican-raised infielder Luis García for Team World, selected to play in the All-Star Futures Game at Nationals Park in 2018. Kieboom began the 2019 season with the Fresno Grizzlies of the Class AAAPacific Coast League. On April 26, 2019, the Nationals purchased Kieboom's contract and promoted him to the major league roster for a series against the San Diego Padres. He made his debut that night and hit a home run for his first major league hit. Kieboom was named to the 2019 Futures Game. Kieboom played 11 games with the Nationals with a.128 average and 2 home runs. The Nationals finished the 2019 year 93-69, clinching a wild card spot and eventually winning the World Series over the Houston Astros. Kieboom did not participate in any postseason action but still won his first world championship. Following the departure of veteran third baseman Anthony Rendon after the 2019 season, Nationals manager Dave Martinez said in January 2020 that Kieboom would move to third base full time, with the expectation that he would step in as Rendon's permanent replacement in the 2020 season.
