When Mary Ellis Peltz, the magazine's editor, retired in 1957, Merkling became editor, a position he held until he retired in 1974. According to F. Paul Driscoll, current editor of Opera News, Merkling gave the magazine "a new point of view, editorially, visually and philosophically," transforming it from an educational guide of the Metropolitan Opera to an international publication.

Under Merkling's tenure, the magazine's covers featured commissioned artwork from Andy Warhol and Saul Steinberg; writers included Virgil Thomson and Andrew Porter. Coverage was also extended to include reviews of Metropolitan Opera performances.

Merkling married Erica Perl, a Vienna-born artist and Opera News staffer in 1952. In 1974, the couple moved to New Milford, Connecticut, where Merkling was a critic for the Danbury News-Times. He was a founder and chairman of the New Milford Commission on the Arts, on which he served for more than 25 years. Erica Merkling, whose career included stints as art director of Opera News and a children's book illustrator, died in 2003.