Garnett was born in Mauldin, South Carolina to Shirley Garnett and O'Lewis McCullough. In his first three high school years, Garnett played for Mauldin High School. However, during the summer prior to his senior year of high school, Kevin was in the general vicinity of a fight between black and white students. Although not directly involved, Garnett was arrested. Due to the racially charged incident and fearful of being a target, Garnett decided to leave Mauldin. He transferred to Farragut Career Academy in Chicago, Illinois for his senior year of high school. He led Farragut to a 28–2 record and was named National High School Player of the Year by USA Today. He was named Mr. Basketball for the State of Illinois after averaging 25.2 points, 17.9 rebounds, 6.7 assists and 6.5 blocks while shooting 66.7% from the field. In four years of high school, Garnett posted an impressive 2,533 points, 1,807 rebounds and 739 blocked shots. He was named the Most Outstanding Player at the McDonald's All-American Game after registering 18 points, 11 rebounds, 4 assists and 3 blocked shots, and then declared himself eligible for the 1995 NBA Draft. Garnett played alongside Ronnie Fields in high school, who also followed a professional career.

Garnett also excelled in soccer during his high school days.