Achiuwa was born in Port Harcourt, Nigeria, to Nigerian parents of Igbo descent and grew up mainly playing soccer.

[7] He led Montverde with 14 points and 7.2 rebounds per game and helped his team reach the semifinals at GEICO High School Nationals.

He joined his former Amateur Athletic Union (AAU) teammate, Lester Quinones, and the number one player in the 2019 class, James Wiseman.

[17] He earned conference freshman of the week honors on December 23 after recording 20 points and nine rebounds in a 77–49 victory over Jackson State.

[23] On August 6, 2021, the Toronto Raptors acquired Achiuwa and Goran Dragić from the Heat via a sign-and-trade deal in exchange for Kyle Lowry.

[24] On October 20, 2021, on his season debut with the Raptors, Achiuwa had eight points, six rebounds, two assists and a steal in 39 minutes of play in a 96–83 loss to the Washington Wizards.

[28] On October 20, the Knicks announced that Achiuwa suffered a strained left hamstring, missing the beginning of the season for 2 to 4 weeks.