Serrano, owner of long arms, began his career on October 29, 1969 with a four-round decision win over Ramon Laureano. He built a fan base in Puerto Rico, campaigning there for his first 23 bouts, including winning and losing the Puerto Rican Featherweight title v.s Francisco Villegas. For his 24th bout, he traveled to Panama City, where he met future world Featherweight champion Ernesto Marcel, who beat him on points in 10 rounds. That would be his last defeat in a long time.