Martínez grew up in Dorado, Puerto Rico. Not highly regarded as a prospect, he signed with the Mariners as a free agent in 1982, and was given a small signing bonus. He made his major league debut in 1987, but did not establish himself as a full-time player until 1990, at age 27. In the 1995 American League Division Series, he hit "The Double", which won the series and increased public support for Mariners baseball as they attempted to fund a new stadium. He continued to play until 2004, when injuries forced him to retire.