In a newly released clip from the film, the actress delivers her version of the sea witch's famous ballad, "Poor Unfortunate Souls."
The classic, originally sung by the late actress Pat Carroll, was written by Alan Menken and Howard Ashman.
While appearing on "Good Morning America" last month, McCarthy spoke about singing in the upcoming film.
"To walk in anything Pat Carroll has done, you're already just like, let me just try to do it some justice," she said.
"It was so fun though," she continued. "Because it is not my forte for sure, so I really nerdishly practiced really hard, and getting to sing with that orchestra was -- I mean, I think it's a once in a lifetime thing. It was ridiculously fun."
"The Little Mermaid" arrives in theaters on May 26.
WATCH: We played Ask Me Anything with Halle Bailey backstage at 'GMA'