The Washington Nationals selected Kieboom in the fifth round of the 2012 Major League Baseball Draft. He played for the Auburn Doubledays in the Class A-Short SeasonNew York-Penn League in 2012, batting.258 with 20 runs batted in in 41 games. He suffered an injury that required him to undergo ulnar collateral ligament reconstruction, popularly known as "Tommy John surgery" – a rarity for a catcher – in 2013, forcing him to scrap plans to play for the Netherlands in the 2013 World Baseball Classic and to miss virtually the entire 2013 season, although he did play in four games with the Gulf Coast League Nationals in the rookie-level Gulf Coast League late in the year, going 2-for-6 at the plate with one RBI. He thus played a small part in a year in which the Gulf Coast League Nationals finished their regular season with a record of 49-9, giving them an.845 winning percentage that was the highest winning percentage for a full regular season ever achieved by a Minor League Baseball team based in the United States. The Nationals then won all three of their playoff games, defeating the Gulf Coast League Pirates in a single-game semifinal playoff and sweeping the Gulf Coast League Red Sox in the best-of-three league championship series, to become the 2013 Gulf Coast League champions. Kieboom played for the Hagerstown Suns in the Class ASouth Atlantic League in 2014, appearing in 87 games and hitting.309, with 61 RBIs and nine home runs, the first home runs of his professional career. In the fall of 2014, he played for the Mesa Solar Sox in the Arizona Fall League, batting.324 in 10 games with a home run and seven RBIs. He spent 2015 with the Potomac Nationals in the Class A-AdvancedCarolina League, where he played in 71 games and hit.248 with two home runs and 21 RBIs. He returned to the Arizona Fall League in the fall of 2015, playing in 13 games for the Salt River Rafters and hitting.238, with two homers and seven runs driven in. The Washington Nationals added him to their 40-man roster in November 2015.
2016
Kieboom was promoted to the Harrisburg Senators of the Class AAEastern League for the 2016 season, hitting.230 in 94 games with five home runs and 31 RBIs. He went home to Georgia after the conclusion of the Eastern League season, but three weeks later the Nationals promoted him to the major leagues for the first time on September 27, 2016, after an injury to starting catcher Wilson Ramos. Kieboom was on the Nationals′ roster for six days, and he had one plate appearance, drawing a walk and scoring a run in the team's regular season finale on October 2, thus ending his first stint in the major leagues with 1.000 on-base percentage.
2017
During spring training in 2017, the Nationals designated Kieboom for assignment on March 2, then outrighted him to the Syracuse Chiefs of the Class AAAInternational League on March 6 before assigning him to Class AA Harrisburg, where he began the season. After he hit.183 in 19 games with Harrisburg, slugging two home runs and driving in six runs, he was promoted to Class AAA for the first time and spent the rest of the season with Syracuse, appearing in 47 games and hitting.275 with three home runs and 19 RBIs. He finished the 2017 season with an overall batting average for the year of.250, with five home runs and 25 RBIs.
2018
The Nationals invited Kieboom to major league spring training in 2018 as a non-roster invitee. Despite not appearing in a regular-season professional game at any position other than catcher, Kieboom took the field more than once during 2018 spring training as a first baseman. He began the season with Syracuse and appeared in 25 games with the Chiefs, batting.250, hitting a home run, and driving in 10 runs while appearing as both a catcher and occasional first baseman before the Nationals called him up to the major leagues on May 11 after starting catcher Matt Wieters suffered an injury and went on the 10-day disabled list. During the first game of a doubleheader against the Los Angeles Dodgers at Nationals Park in Washington, D.C., on May 19, Kieboom had his first career major-league start, and in the third inning he notched his first major-league hit, singling off of Dodgers starter Ross Stripling. On September 11, 2018, Kieboom hit his first career home run while facing Philadelphia Phillies pitcher Nick Pivetta.
2019
Kieboom spent the 2019 season with Harrisburg, hitting.196/.271/.256/.527 with 1 home run and 14 RBI. On October 21, 2019, the Nationals outrighted Kieboom off their 40-man roster. He elected free agency on October 24.
