Petagine entered professional baseball on February 13, 1990, at the age of 18, when Houston signed him as an undrafted amateur free agent after starting his career in Venezuela with the Leones del Caracas baseball team in which he contributed with his power for several campaigns becoming a star and a well-known baseball player in his country. Later that year, he played his first pro season with the rookie-level GCL Astros of the Gulf Coast League. His production there, and the following year with Single-A Burlington, was solid (if unspectacular). Petagine would go on to substantial minor league success in his early to mid 20s, but even his early stats hinted at the trends that would define—and perhaps curse—his career in American baseball: always having an above-average OBP, but relatively little home run power, by the standards historically applied to first basemen.