There were several conspiracy theories before the internet. There was the idea that FDR allowed Pearl Harbor to be bombed gained some traction, for example. The John Birch Society advocated the idea that communists had infiltrated our government (and was putting fluoride in our water! - those damn commies!). The McCarthy hearings (1954) were an expression of that conspiracy theory, but it was only a public demonstration of a popular undercurrent of a conspiracy-minded movement.

More importantly, the two Kennedy assassinations were the subjects of endless books and enthusiasts who imagined all sorts of possibilities. Similarly, the idea that the government was restricting access to evidence of alien visits to earth was enormously popular. All of these thrived well before the internet, and the common denominator tended to be that the government was doing things against the will of the people, often to harm the general population.

What one did not have before the internet was the explosion of multitudes of conspiracy theories. There are now so many different ones, that is it hard to keep track of all the possibilities.