Robertson attended Exeter High School before being drafted by the Astros in 1995. He spent the next seven years moving up the minor league system before making his debut in September 2002. His most productive year was 2003, where he won 15 games and finished seventh in Rookie of the Year voting. He was traded to the Indians the following year, where he played in eight games before being demoted to the minor leagues. Robertson spent the next few years playing in various Minor League and independent baseball organizations, including the Mexican League, Chinese Professional Baseball League, Puerto Rican Professional Baseball League, and Venezuelan Professional Baseball League.