Who sang the original Grinch song from 'How the Grinch Stole Christmas'?

BORIS KARLOFF

It was definitely Boris Karlof who also narrated it

Sorry, it was Thurl Ravenscroft...voice of Tony the Tiger.

In 1994, a special edition of the original cartoon classic aired on TNT (Turner Network Television). Narrated by the late Phil Hartman, an extra 20 minutes was added for this special with several "behind-the-scenes" looks at the animation, the making of the cartoon, and special interviews, including Tim Burton. It also featured the late Thurl Ravenscroft, the non-credited singing voice of the grinch and the voice behind, "You're a Mean One, Mr. Grinch." Ravenscroft explained the oversight, caught after the film was presented to the studio for airing, left him off the closing credits of the original short cartoon. He is credited at the end of the special edition.

Retrieved from :
https://en.wikipedia.org/wiki/How_the_Grinch_Stole_Christmas_%28animated_short%29

Thurl Arthur Ravenscroft (February 6, 1914 – May 22, 2005) was an American voice actor and singer with a deep, booming voice. For 53 years, he was best-known as the voice of Tony the Tiger in more than 500 television commercials for Kellogg's Frosted Flakes (also known as "Frosties"). After his death, Lee Marshall replaced him as the voice of Tony the Tiger in the Kellogg's commercials.

Ravenscroft was the vocalist of the song "You're a Mean One, Mr. Grinch" in the Christmas television special based on the Dr. Seuss classic How the Grinch Stole Christmas! His name was accidentally left off of the credits, however, leading many to believe (erroneously) that the cartoon's narrator, Boris Karloff, sang the song. Ravenscroft also sang No Dogs Allowed in the Peanuts animated motion picture Snoopy, Come Home.

https://en.wikipedia.org/wiki/Thurl_Ravenscroft