Born in Eugene, Oregon, he is the son of Florian Hartenstein, a German basketball coach and former professional basketball player of Afro-German descent. Isaiah's mother is American. His parents met in the United States when his father attended the University of Oregon. In 2008, Hartenstein and his family moved to Germany, where his father was playing professionally.
Professional career
European career
Hartenstein joined the MTV Gießen youth ranks and continued his career at the youth teams of QTSV Quakenbrück/Artland Dragons, after his father had signed to play with the Artland Dragons in 2009. In the 2013–14 season, Hartenstein led the Artland Dragons' junior team to a German championship in the Under-16 Bundesliga JBBL, while being named Most Valuable Player. He averaged 20.9 points, 12.1 rebounds, 3.4 assists, 2.9 steals and 1.9 blocks on the season. His father Florian who served as head coach of the team was named JBBL Coach of the Year. Hartenstein was selected to play in the 2014 Jordan Brand Classic International Game: He finished the game with four points and five rebounds in 18 minutes. Hartenstein helped the Žalgiris Kaunas Under-18 squad to win the qualifying tournament to the Euroleague Basketball Next Generation Tournament and was selected MVP. On February 1, 2015, Hartenstein made his debut in Germany's top-tier levelBasketball Bundesliga, seeing 1:12 minutes of action against Eisbären Bremerhaven. In August 2015, Hartenstein inked a deal with Lithuanian powerhouse Žalgiris Kaunas, but he remained with the Artland Dragons on loan. The Dragons had withdrawn from the Bundesliga, and were relegated to the 2. Bundesliga ProB, Germany's third division. Hartenstein played in 14 games for the Quakenbrück based side during the 2015–16 campaign, compiling averages of 11.6 points, 8.9 rebounds, 2.1 blocks, 1.6 steals and 1.4 assists. In January 2016, he decided to leave the Artland Dragons to join Žalgiris Kaunas. Hartenstein made his debut for Žalgiris in Lithuania's top-flight league, the LKL, on September 28, 2016 against Šiauliai and in the EuroLeague against Fenerbahce on October 26. Later in that season, he helped Žalgiris win their first ever King Mindaugas Cup.
In the 2017-18 campaign, Hartenstein appeared in 38 games of the NBA G League, averaging 9.5 points and 6.6 rebounds a contest for the Rio Grande Valley Vipers. On February 7, 2019, Hartenstein recorded his first career triple-double after posting 12 points, 16 rebounds and 11 assists in a 103-102 road win against Salt Lake City Stars. In the 2018–19 season, Hartenstein scored 33 points and made a career-high 8 3-pointers in the series-clinching game as they won the NBA G League championship with the Rio Grande Valley Vipers. He won the NBA G League Finals MVP award.
Houston Rockets (2018–2020)
On July 25, 2018, the Houston Rockets signed Hartenstein. After being sent down to the Rio Grande Valley Vipers, he was suspended one game without pay after leaving the bench during an altercation in a 132-109 loss to the Memphis Hustle on December 17, 2019. On June 23, 2020, the Houston Rockets announced that they had waived Hartenstein.
