Martínez made his professional debut in Minor League Baseball with the Bellingham Mariners of the Class A-Short Season Northwest League in 1983 as a third baseman. He had a .173 batting average, zero home runs and only 18 hits. The scout who signed Martínez convinced Hal Keller, the Mariners' general manager, to assign him to the Arizona Instructional League (AIL) after the season. Keller did not believe Martínez would be able to hit in the major leagues, and initially did not want to assign him to the AIL, which is reserved for the best prospects. Keller included Martínez in the AIL that year, where he batted .340.