Marcus William Giles was born on May 18, 1978, in San Diego, California. He has an older brother named Brian. As a kid, Marcus could never sit down for very long. He was always dragging his dad out to practice. He never wanted to play baseball professionally, just wanted to do what his older brother did.

Marcus attended Granite Hills High School, where he played baseball and football. He was on the All-Star team and graduated in 1996. He then attended Grossmont Junior College, where he excelled in baseball. He was All-Player of the Year in 1997.

Marcus was picked in the 53rd round of the 1996 draft. He was sent to somewhere then to Richmond.