“The Yellow Kid” wrongly gets credit for a couple of things. It’s often cited as the first “panel” cartoon, but that’s wrong; several toons (including “Hogan’s Alley”, where the YK got his start) pre-dated it. Also, the YK wasn’t the first to use word balloons, because he didn’t use them at all! The Kid’s comments always appeared on his shirt, which would change dialogue from panel to panel.
I know that American political cartoons used an awkward version of the word balloon in the early 19th century; I recall seeing a cartoon criticizing then-President Andrew Jackson that used balloons. The script was very tiny, and the writing verbose, which made the balloons almost unreadable. Mid-19th century cartoonist, like Thomas Nash, used almost “silent” cartoons in order to make their point more effectively.
As pointed out above “fumetti” is the usual term for such balloons. IIRC, there was even a magazine devoted to comic art called “Fumetti”.
One book on comics history extended the use of such devices back much farther than has been suggested here – in the Middle Ages drawings and tapestries sometimes depicted the words of a character written on one-line scrolls near their mouths.
There has been a lot of variation in the uses. I recall a 1950s Batman comic that indicated whispering by placing “(whisper)” at the start of the dialogue in the balloon. Sometimes they’d place the dialogue inside parentheses in the balloon. In fact, the artist is only limited by his imagination and what he can get away with in conveying his meaning to the reader. I’ve seen robots talking in “computer voices” indicated by those distorted “optical characters”. The French strip “ASterix” has done some very clever stuff with word balloons. And playing with the items in those balloons goes back long before Walt Kelly – it was a staple of animated cartoons for years, and goes back at least to Otto Messmer’s “Felix the Cat”, who’d use his fumetti question mark as a fishhook, for example.
Well I couldn’t find anything definitive, but the consensus
among cartoonists I know was they were popularized in
the late 50’s/early 60’s in their modern form, with Stan
Lee using entirely too many of them.
Remember that Snoopy didn’t “think” for many years at the
beginning of Peanuts, so this late 50’s timeframe for
standardization fits in there too.
I would be interested, of course, in examples that take them
back earlier.