Roy is an Army brat who was born at Fort Lewis, Washington. His father was a soldier in the U.S. Army. His family lived in Fort Eustis, Virginia from 1989–1996 before moving to Killeen, Texas, when Miller was in middle school. Miller was a star at Shoemaker High School before moving on to the University of Texas at Austin. Miller was an All-American, two-time All-State and three-time All-District performer at defensive tackle and also played some offensive tackle. The All-Area Defensive MVP recorded 111 tackles with 10 sacks, 17 stops for loss, 19 pressures, six pass break-ups and two fumble recoveries, returning one 43 yards for a touchdown as a senior at Shoemaker. In three seasons, the team captain collected 264 tackles with 24 sacks. Roy was a prep All-American who earned practice MVP honors at the U.S. Army All-American Bowl.