Romo, who was nicknamed "Huevo" (meaning Egg in Spanish), began his professional career with the Tigres de Aguascalientes in the now-defunct Mexican Center League.

After three seasons in the minor leagues, mostly with the Portland Beavers Romo was selected in the Rule 5 draft by the Los Angeles Dodgers.

During the last three months of the season, Romo appeared in 40 games for the Indians, posting a team-best 12 saves and a 1.62 earned run average.

Romo was traded along with Sonny Siebert and Joe Azcue from the Indians to the Boston Red Sox for Ken Harrelson, Dick Ellsworth and Juan Pizarro on April 19, 1969.

In 1971, Romo was traded at the end of spring training to the Chicago White Sox, where he spent two seasons as a middle reliever.

Four days later, Romo made his first major league appearance in eight years, pitching two shutout innings against the Chicago Cubs.

After one relief appearance, he pitched seven scoreless innings against the Montreal Expos on July 19 for his first major league victory since 1974, and first as a starter since 1970.