Rountree was born in Los Angeles, California. When he was two years old, his father was shot and killed in an attempted robbery while working as the tour manager for Boyz II Men. Growing up overweight and self-described as "non-confrontational," Rountree was in a band traveling the country. Eating unhealthy and smoking cigarettes, he decided to begin training after watching The Ultimate Fighter with his brother. At the age of 20 and weighing 300 lbs., Rountree joined Wand Fight Team gym in an attempt to lose weight. After one year of MMA training, Rountree started his MMA amateur career in 2011. Khalil trains at Syndicate MMA Las Vegas. Before his UFC 236 fight against Eryk Anders, Rountree trained in Thailand, focusing on Muay Thai.