What was the first product to receive Kosher certification?
The Gemara (Bab. Shabbath 21a) tells us that this practice, on some level, dates way back at least to the time of the Ḥashmonaim. the Kohanim would light the Menorah in the Beith HaMikdash using oil in sealed containers bearing an official seal that, unless tampered with, marked that the oil was pure and usable for the Menorah.
Modern-day kashrus certification hasn't been around for that long. The honors for "first to be certified" under the current system would seem to belong to Heinz Vegetarian Beans, which in 1923 became the first product to carry the OU symbol. The OU is the oldest certifying agency.
I believe there was a proto-OU organization that certified Sunbeam crackers. There's an ad for them in Yiddish with celebrity endorsement by -- Yossele Rosenblatt! (See link -- the Rosenblatt ad was with OU, but I recall hearing that Sunbeam hechsher predates the OU. The PDF was too fuzzy to read the ad's text.)
There's also Rabbi Tobias Geffen's famous responsum on Coca-Cola, which involved some reformulating to make it kosher. But that was an individual rabbi not an organization?
There have been products with hechsher in the past, but not as formalized as today. In the late 1700s the Chayei Adam writes that the two ports of arrival for sugar in Europe were Hamburg and Amsterdam -- a mashgiach would meet the boat and seal some packages of sugar, "kosher for Passover" -- as the sugar you'd usually buy in Europe may have been adulterated with flour.
There is some evidence that the Jewish population of Rome had Kosher supervision 2,000 years ago. See this article: The Garum Debate: Was There a Kosher Roman Delicacy at Pompeii?