That woulda been Fritz the Cat, I believe.
Paul Lynde, well-known for much of the 50s, did lots of animation voices starting circa 1969.
Mel Blanc was well-known outside his animation work, but probably after that had been going a while.
I’m sorry, but the OP doesn’t make a whole lot of sense to me. Isn’t it *actors *who supply the voices for cartoons? Aren’t some actors celebrities, specifically *because *they’re actors? I mean, the OP seems to suggest that the standard must be to cast only unknowns. I don’t see very many of the examples given in this thread as any kind of remarkable casting; these actors were offered a part like any other, no? Some of them were chosen because they had marquee value, but how is that different from any Hollywood casting?
I get the OP - at one point you didn’t hire celebrity actors for your roles. At some point that changed (although I’d argue that most of the Disney modern classics have unrecognizable names to most people in lead roles - the big exception being Robin Williams - but I wonder how much of that part was expanded given his ability to improve and the amount of good material he gave them.)
However, I don’t get why having a recognizable voice ruins the movie. I don’t go to a Tom Hanks film and say “hey, that’s Tom Hanks on the screen” so I don’t go to a Tom Hanks animated film and say “hey, that’s Tom Hanks’ voice.”
My understanding is that voice work for celebrity actors is fun and pretty easy - compared to a normal shooting schedule, they are in and out pretty fast. So they want to do it. It adds names to the marquee, so the animation companies like it, too.
RC, yer makin’ too big a deal over one example I cite (as only a possibkle case) while I list lots of others that undeniably fit the OP.
That was unquestionably the beginning of the current obsession with name actors in animated roles. There was a huge amount of press around Williams’s performance as the Genie, and it helped Aladdin’s box office take. There had been name actors in animated roles before but the current state - where virtually ALL animated films are anchored by name stars - started with Aladdin.
Whether or not this is hurting the genre artistically… well, I can’t see that. Animated films either are good or they suck or they’re somewhere in between. The “Toy Story” films have major stars, but are two of the finest movies Hollywood has put out in the last twenty years, because they’re of such high quality - great scripts, great animation, great “direction,” and yes, great voice acting. On the other hand, “Over the Hedge” was a mediocre, by-the-numbers effort, and “Ice Age” was just painfully bad. All those films had name actors.
Every Pixar film has name stars; Toy Story I/II has Hanks and Allan and several other very recognizable voices like Don Rickles, Kelsey Grammer, and Joan Cusack. The Incredibles had Holly Hunter and Samuel L. Jackson, plus several second-tier stars like Craig T. Nelson and Jason Lee. Finding Nemo had Albert Brooks, Ellen DeGeneres, Willem Dafoe, Brad Garrett, Geoffrey Rush, and Elizabeth Perkins. Cars had Owen Wilson, Paul Newman, Cheech Marin, Bonnie Hunt, etc. etc.
I meant, every company besides Pixar seems to give the celebrity actors more billing/focus than the script and animation. I am speaking generally, but you’re welcome to challenge that if you like, because I’d be happy to find the exceptions 
Of course, many of the Christmas specials made for TV in the 1960s had a big-name as a narrator: Burl Ives, Jimmy Durante, Fred Astaire, Boris Karloff, etc.
For the record, some early Betty Boop cartoons had Cab Calloway performing in them.
I agree with the OP. Actors may still be actors, but they are not voice actors which I think is a fine difference. In some cases it does not matter, in others it does. By hiring big names only I think you are neglecting what has been a fine tradition of animated features.
Other popular performers as well. One of them even has a very young Louis Armstrong (His face even gets into the regular cartoon – the only time I know of that that happens)