According to ISFDB Leinster's "First Contact" is a novelette, not a novella. I guess they use the SFWA standards: 7500 to 17500 words for a novelette, 17500 to 40000 words for a novella. By the way, the story "First Contact" is not exactly the first occurrence of the term first contact in a stfnal context, but I think I'll post that as an answer.

@user14111: Thanks! I have updated the answer in regards to both of your points. :-)

@Praxis: The random occurrence of the two words in sequence in your first two items is not particularly useful. But the 1916 one is absolutely unequivocal. You have the phrase used almost as a standalone, with the right meaning, clearly being used for more than the sum of its constituent words. Nice.

@ThePopMachine: Glad to be of service! (The first part was really just me having some fun with the Ngram Viewer.)

@ThePopMachine The difference between this answer and the others is that these are examples of the phrase "first contact" being used in a non-sci-fi sense; they mean first contact between two human cultures. I assumed, since this is SF&F, you meant first contact with aliens :)

As for Google Ngram: I think you should have searched for first contact (no quotes), with case insensitive checked - this shows results since 1705 - due I doubt they are on-topic or have the same meaning.

@MichaelEdenfield Well, the "human cultures" were considered aliens. In some cases, the other people weren't even considered human (because of course, how could a human not be a christian, right?). This makes it quite analogous to the use in especially Star Trek, where the different aliens seem to be just different races, not even species (especially from TNG on - TOS still worked with Spock's hybrid question as something assisted by technology; it has grown into pretty much free breeding between the aliens over time).

@MichaelEdenfield: I think it's pretty clear from the original question what my intent was and why this answer is correct. A question about a term used commonly in SF where the answer might be "actually, this term predated SF" is totally in bounds. Just like the answer to some Star Trek questions is "the has it roots in naval tradition for hundreds of years". All that having been said, if someone wanted to synthesize the other answers into this one, it would make a more complete answer.

@ThePopMachine : I have synthesized the additional information from the other answers into this one, as requested. :-)

What is the exact reference for that 1916 Lucy Thompson quotation? (Edition, page number). Are you sure it was from the 1916 edition? Haven't been able to find it.

@user14111 : I don’t have the text, but I would presume so. I was relying on Google Ngram’s records, as stated above.

Sorry for being a nuisance. The reason I'm asking is, I passed on your citation to an OED editor, and he replied: "I agree that the passage quoted in the Stack Exchange entry represents the sense we want, but I'm unclear where it comes from. It's not in the 1916 book that the entry links to, it's not found in any other book in Google Books, and the only example of the passage I can find on Google is from the Stack Exchange entry itself." So I spent some time searching Google Books for it myself, but of course I didn't find it either.

@user14111 : All I can say is that, 5 years ago, there must have been a Google Books excerpt with that quote (I’m not sure where else it would have come from). Google Ngram pointed me to the 1916 edition of this book and I used Google Books (I believe) to get the exact quote it referred to. I wish I knew more, but this is the best I can say.

@user14111 : If I go to what appears to be the 1916 version of the text in Google Books and try the same text search, it comes with a hit on page 175 but does not display the full text: books.google.ca/… Is it possible it is searching text from a different version (it says 1916), or perhaps there is some error? The text definitely came from Google Books 5 years ago, and it still seems to say that the phrase is in that particular edition.

@user14111 : I’m wondering now if it was part of a foreword, etc., perhaps by someone else, that was not part of the text proper, and Google is no longer including it with the text.

@user14111 : As I’m no longer confident about this, I have revise the answer accordingly. (If you manage to solve the mystery, I’ll revise the answer again.) In the meantime, I hope your OED editor will find a different and suitable example. –

@user14111 : Makes sense, thanks for the context. Sorry I can’t be of more help reconstructing this right now.

@user14111 : In any event, I would ask them not to use that quotation until a source and correct date can be found.

I searched for "first contact" with quotation marks and got one hit, on p. 175. There's a "first" and a "contact" but no first contact. The quotation marks were useless. google.com/books/edition/To_the_American_Indian/…

Don't worry, the OED people don't shoot from the hip. They check everything very carefully.

@user14111 : Thanks. I don’t think the original search had a hit on page 175 anyway. I think the quoted appearance was much earlier in the text. I’m definitely starting to think it was in a foreword that is no longer bundled with that source and which was impossible to distinguish from the main text under Google Books’ “Snippet View”.

An earlier non-SF use of the phrase can be seen in the Southern Quarterly Review, Volume 4 from July 1843, which speaks of "the results of the first contact of Roman and barbarian society".

Also, astronomical uses of the term for eclipses go back even further, see p. 1104 of Philosophical Transactions of the Royal Society of London from 1670.

On the issue of quote that was originally thought to be from the 1916 Lucy Thompson book, I think it may actually have been from Julian Lang's modern introduction to the 1991 edition of the book, the quote "By 1916 the Indian cultural landscape was covered with..." that Praxis originally included in the answer is given as part of Julian Lang's introduction on p. 51 of the thesis at escholarship.org/content/qt1365d8cr/qt1365d8cr.pdf