[4][5] He spent his first professional season with the rookie–level Gulf Coast League Nationals where he batted .244 with four home runs and 25 RBIs.

[7] Kieboom was named to the Northern Division All-Star team in the South Atlantic League, alongside several Suns teammates.

[18] 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.

The Nationals finished the 2019 year 93-69, clinching a wild card spot and eventually winning the World Series over the Houston Astros.

[23] In 2020 for the Nationals, Kieboom played in 33 games, hitting .202/.344/.212 with no home runs and 9 RBI before ending his season early due to a left wrist contusion.

On March 20, 2022, it was announced that Kieboom would miss 4-6 weeks while recovering from a flexor mass strain near his right elbow, and he was placed on the 60-day injured list the following day.

[25] Kieboom began the 2023 season in extended spring training before being activated by the Rochester Red Wings of the Triple–A International League in May.