Mayo began playing high school basketball at Rose Hill Christian High School in Ashland, Kentucky. In Kentucky, grade schoolers can play high school basketball, and Mayo averaged 23.1 points for Rose Hill's varsity team while in 7th grade. In 8th grade he was just turning 15 and tallied 27 points per game and was named a first-team all-state player by the "Louisville Courier-Journal". Mayo then moved to suburban Cincinnati to live with club team coach Dwaine Barnes. (Mayo referred to him as his "grandfather" but the two aren't related.) Mayo enrolled at North College Hill High School in April 2003. The "Cincinnati Enquirer" and local television stations sent reporters to cover Mayo's first day of school at NCH.