I was watching TV and I saw an annoying bit of product placement which got me thinking- what was the first product placement? Was there some play where the characters started talking about the quality of some product because they were paid to do so, or is this a modern phenomenon which only arose from film and television?
Just to be clear, I am asking about the advertisement of products in a work which wasn't created with this as the intent, so I am not asking about medicine shows and related events.
I'm almost certain the answer is tobacco. Long before product placement was common with products like beverages and fashion items the tobacco industry had mastered it as an art. They had of course long been involved with advertising and sponsorships in tv and movies, but they quickly learned that merely depicting tobacco use by fashionable or admirable characters or actors in movies was incredibly effective, and brands could be easily made visible as well.
The biggest factor pushing the development of product placement of tobacco was that in 1970 tobacco advertisement on tv was banned in the US. This despite tobacco companies being the leading advertisers and sponsors of tv content at the time. So instead they switched to relying on product placement, which was unregulated. That wasn't when they started doing product placement, just when they began to rely on it more heavily.