There have been a couple interesting threads on this in the past that you might want to check out!
It would be easier to document when making out started to be called making out than when they started doing it, anywhere, period. But at least by the 1920s, "petting" was a recognized American phenomenon embraced by some and condemned by others, and by the 1940s and 1950s "necking" between partners in a committed relationship seems to have been broadly at least tolerated. If the changing presence of making out in dating culture interests you, Beth Bailey's excellent book From Front Porch To Back Seat documents both shifting dating practices in North America and shifting attitudes toward kissing, petting, and sexual conduct within the framework of dating. It's jarring to reach a point in the book where an early 60s writer advises young men to coerce their partners into first kissing, then petting, and ultimately sex as sort of a "for their own good" maneuver, reasoning that sex drives are healthy and normal even outside of marriage and with no intention of culminating in marriage and that girls just need to be firmly persuaded into accepting this fact about themselves. (While the equivalent dialogues for women are still cleaving largely to the "nice girls don't let it go too far" narrative where making out and heavy petting are on the edge of acceptability.) But still, regardless, making out was clearly happening all through the 20th century -- it was attitudes about those acts under the heading of "making out" (what kind of people did them, their moral acceptability, how far you can go, whether they were inherently a prelude for sexual intercourse or a smokescreen for it) that reveal a lot and are considerably easier to document than ephemeral evidence of a deed that generally involves only two people in a reasonably private location.
There's a lot of other stuff to unpack in this question too -- when's the earliest documentation we have for partners swapping erotic kisses? What's the earliest documentation we have for kissing and embracing as the main act of a sexual encounter rather than as a prelude to sexual intercourse? How about specifically as a prelude to sexual intercourse? Between casual partners? In semi-public places? When and where was "making out" with a partner socially tolerated or winked-at in mainstream respectable society? For my two cents, the thing that's most striking about kissing within my historical timeframe of familiarity (late Medieval/Early Modern) is how much of it isn't tongue kissing -- kisses of peace, kissing lepers, tearful embraces with kisses between male friends, otherwise symbolic kisses -- as well as instances that are clearly erotic-romantic kisses where one might reasonably expect a little tongue action. The whole constellation of activities such as embracing, fondling, intimate conversation, even mutual masturbation/frottage, and other activities I associate with modern "making out" can be documented in the Early Modern period -- sometimes under adverse circumstances (16th and 17th accounts of proceedings surrounding sodomy, for instance, where it's important for the prosecution to determine whether the "complete act" of penetration and ejaculation occurred but pleading "hey, he and I just made out!" wouldn't necessarily save your hide either) and sometimes more positive ones.