He is the son of Florian Hartenstein, a German basketball coach and former professional basketball player. His parents met in the United States, when his father attended the University of Oregon. Isaiah started playing basketball in the United States, in 2008 he and his family moved to Germany, where his father was playing professionally.
Isaiah"s mother is American. Isaiah joined the Music Television 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 2013-2014 season, Hartenstein led the Artland Dragons" junior team to a German championship in the 16-under 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.
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. On February 1, 2015, he made his debut in Germany"s top-tier Basketball Bundesliga, seeing 1:12 minutes of action against Eisbären Bremerhaven.
In August 2015, Hartenstein inked a deal with Lithuanian powerhouse Žalgiris Kaunas, but 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 The Dragons had withdrawn from the Bundesliga and were relegated to the 2.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 2015-2016 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. In February 2016, he attended the "Basketball Without Borders Global Camp" during the National Basketball Association Allstar weekend in Toronto, Canada.
Hartenstein represented Germany at the 2014 U16 and the 2015 U18 European Championships.
