While living in Brownsville, López began working various jobs to bring in income and never returned to school. As a young man, López caught the attention of a boxing promoter, and for seven years he traveled the country fighting a total of 55 fights in the lightweight division with the nickname of "Kid Mendoza". In 1934, during a boxing match in Melbourne, Victoria, Australia, he met a group of Merchant Marines and signed a contract with them. He was accepted in the union in 1936 and spent the next five years traveling the world.