Searching for a replacement for Costello, Lou Fleischer heard Mercer singing the Popeye theme song and gave him the job of doing the voice.

For Fleischer's feature film Gulliver's Travels he voiced King Little, Twinkletoes the carrier pigeon, and the bumbling spies Sneak, Snoop, and Snitch.

Beginning in 1958 Mercer also provided all the voices for the Felix the Cat TV cartoons, including The Professor and Rock Bottom, produced by former Paramount animator Joe Oriolo.

He wrote hundreds of scripts for various cartoon series at Paramount, including a number of Popeye episodes, as well as for television's Deputy Dawg and Milton the Monster.

[6] Originally a resident of New York City, Mercer moved to Miami, Florida, when Fleischer Studios relocated there in 1938.

[citation needed] He died at Lenox Hill Hospital in Manhattan on December 7, 1984, after stomach cancer-related problems.