Nootbaar was born to an American father of Dutch descent, Charlie Nootbaar, and a Japanese mother, Kumiko Enokida.[1] Nootbaar grew up in El Segundo, California.
Nootbaar began the 2021 season at the Cardinals' alternate training site before being reassigned to the Triple-A EastMemphis Redbirds.[13] He was placed on the injured list with a hand injury on May 28, and was activated on June 14.[14]
On June 22, 2021, Nootbaar was selected to the 40-man roster and promoted to the major leagues for the first time.[15] At the time of his promotion, he had a .329/.430/.557 slash line with five home runs and 17 RBIs over 22 games.[16] He made his MLB debut that day as the starting left fielder against the Detroit Tigers.[17] The following day, Nootbaar recorded his first career hit, a triple.[18] He hit his first career home run as a pinch-hitter off JT Brubaker in a 7–6 win over the Pittsburgh Pirates on August 12.[19] Nootbaar hit another pinch-hit home run the next day in a 6–0 victory over the Kansas City Royals.[20] On August 25, Nootbaar got his first career walk-off hit, a single in the 10th inning against Tigers relief pitcher Michael Fulmer.[21] Nootbaar finished the 2021 season slashing .239/.317/.422 with five home runs and 15 RBIs over 109 at-bats.[22] He was selected to play in the Arizona Fall League for the Glendale Desert Dogs after the season.[23]
Nootbaar entered the 2022 season as the Cardinals' fourth outfielder before eventually moving into a starting role after injuries and positive play. Over 108 games and 290 at-bats for St. Louis, he hit .228/.340/.448 with 14 home runs, 40 RBIs, and 16 doubles.[24] Nootbaar mainly played in center field for the Cardinals in 2023, also spending time in left and right field. Over 117 games, he batted .261 with 14 home runs and 46 RBIs.[25]
In 2006, a Japan national youth team toured the US, including future MLB player Masahiro Tanaka and Japanese high school pitching phenom Yuki Saito.[26] Nootbaar served as a batboy and interacted with the team, including stretching and playing catch, and some of the players stayed at his house and shared meals with the family.[26] According to his mother, this experience was later influential to Nootbaar accepting an offer to join the Japan national team for the 2023 World Baseball Classic. The team went on to win the tournament, with Nootbaar batting leadoff and recording an RBI in the gold-medal match against the United States national team.[27][28] Nootbaar was the first player not born in Japan to represent the country in a World Baseball Classic.[29]
During the 2022 season, Nootbaar became a fan favorite among Cardinals fans, often being greeted with "Nooooot!" when batting or making a defensive play, which may be mistaken for booing.[32]