Mobley attended Rancho Christian School in
Temecula, California. As a freshman, he won the
CIF Southern Section (CIF-SS) Division 5A title, the school's first in any sport, and led his team to the CIF Division V Southern California Regional final. After averaging 16.2 points and 10.4 rebounds per game, Mobley shared CIF-SS Division 5A player of the year honors and made
The Press-Enterprise All-Area second team. Over the summer, he played for the Compton Magic, one of the top travel teams in the country. As a sophomore, Mobley was joined on the Rancho Christian basketball team by his younger brother
Evan Mobley. He helped his team reach the CIF-SS Division 2A semifinal. In his junior season, Mobley averaged 19.9 points, 11.3 rebounds, and four assists per game and was named
The Press-Enterprise player of the year and made the
USA Today All-USA California second team and
MaxPreps Junior All-American honorable mention team. He guided Rancho Christian to a 29–5 record and a CIF-SS Open Division playoff appearance. As a senior, Mobley averaged 19.4 points, 13.6 rebounds, and 3.8 assists per game, helping his team to a 26–6 record. He earned honorable mention on the MaxPreps All-American and
USA Today All-USA teams, while making the All-USA California first team. Mobley played in the
2019 McDonald's All-American Game. Mobley at one point in high school was projected to be the second pick of the
2020 NBA draft and his brother
Evan Mobley was projected to be in the
2021 NBA draft.
Recruiting Mobley received offers from several
NCAA Division I programs, including
San Diego State and
Nevada, before starting high school. On May 28, 2018, as a high school junior, he committed to playing college basketball for
USC. By the end of his high school career, Mobley was considered a consensus five-star
recruit and the best 2019 class prospect in California. ==College career==