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.

[8] On February 1, 2015, 16 year old Hartenstein made his debut in Germany's top-tier level Basketball Bundesliga, seeing 1:12 minutes of action against Eisbären Bremerhaven.

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.

[18] 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.

[20] 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.

[21] 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.

[22] 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.

[31] On November 20, 2024, Hartenstein made his Thunder debut, putting up 13 points, 14 rebounds, five blocks, and three assists in a 109–99 win over the Portland Trail Blazers.

[34] He helped carry Germany to a fourth-place finish at the 2016 FIBA Europe Under-18 Championship, averaging team-highs of 14.7 points, 9.5 rebounds, 1.7 blocks, and 1.7 steals a contest,[35] which earned him a spot in the tournament's "All-Star Five".