Well, it just so happens that I bumped into an article from a 1983 issue of American Speech where the answer was to be found (please don’t ask how I seemed to be randomly browsing such an esoteric journal).

Philip Kolin says: The coinage mocktail appears for the first time, I believe, in an advertisement for Libbey Glass in Food Service Marketing (Feb. 1979, p. 76). According to that ad, mocktails “are a relatively new group of beverages prepared without any alcohol whatsoever.” Kolin comments that the ad was for a new line of mocktail glassware (I had no idea that glassware was so specialized – perhaps because most of mine was purchased at Ikea).

He further states, clearly with the air of someone who loves language: Mocktail is a clever invention. It humorously rhymes with cocktail, but has a semantically appropriate first…