His father Bob Horner was Mason City High School varsity basketball coach.
In his senior season, Horner averaged 31.2 points, 8.7 rebounds, and 6.8 assists per game, leading the prep team to a 21–2 overall record.
He shot 53% from the field and 73% from the free throw line as a senior, averaged 25.3 points a game as a junior, along with 10.1 assists and 5.2 rebounds.
He averaged 21.2 points, 10.2 assists and five rebounds per game as a sophomore, and led Mason City to 2 state tournament appearances as a junior and senior, with a 38–7 record over those two seasons.
[4] Horner was the head varsity basketball coach at Valley High School in West Des Moines, Iowa, from 2010 to 2014.