William Hartson called the word “surely one of the ugliest words ever to slither its way into our dictionaries”, but regardless of what he would like to say about the word, I actually have always found it quite delightful, and others must have thought so too since seeing as it became so widespread.
I have heard in sources that it derived from college slang, and Online Etymology Dictionary puts:
also humungous, by 1972, American English, apparently a fanciful mash-up of huge and monstrous.
I found the following use in 1969 in "Princeton Alumni Weekly, Volume 70."
The response was humongous ( Gargantuan in size . Bigger than big )
In the example since the author is seen to feel a need to define the word surely leads one to assume that the word was exceptionally infantile to the English tongue.
In a 1967 book titled Current Slang the word is defined:
Humongous , adj . Heavy , large
And it looks as though by 1970 the word was being used freely and does not infer that a reader might be unfamiliar with the word's meaning.
I cannot find any records on who coined the word, and if so, where did it first appear? It is such a "new" word, that I doubt there cannot be found the first coining of it.