I just looked it up, and I was not completly correct. It was just the song:
Boris Karloff, best known for his 1931 portrayal of Frankenstein, narrated and provided the voice in the 1966 version of Dr. Seuss' How The Grinch Stole Christmas. But the classic solo of You're a Mean One, Mr. Grinch (and all other musical numbers attributed to the Grinch) were performed by Thurl Ravenscroft, the same voice who gives Tony the Tiger his Grrrreeeeaat! sound. Ravenscroft was overlooked when the credits for the movie were added, causing concern for Theodor Geisel, who made a personal attempt to publicly note the error.
And remember Cindy Lou Who? Voiced by June Foray, she also provided the sound of Rocky the Squirrel in both the 1960's cartoon series and in the recently released live action movie.