What was the first prime time animated (cartoon) series on American television?

The first prime time animated (cartoon) series on American television was the Flintstones (1960-1966). The show's gimmick, besides being a rare prime time cartoon, was that the Flintstones family had everyday modern situations set in prehistoric times.

There was no electricity, no internal combustion engines, and no shoes, but the characters still had modern conveniences like dishwashers and phonographs. All these appliances were powered by captive animals (including dinosaurs). The heavy equipment Fred and Barney worked with was powered by pulleys, and their cars were powered by the courtesy of two feet.

This show clearly provided several breakthroughs for network television. It was the first animated show that addressed the issue of infertility (the Rubbles adopted). Also, the Flintstones (Fred and Wilma) were the first couple to sleep together in a cartoon.

In 2013, TV Guide ranked the Flintstones the second "Greatest TV Cartoon" of All Time.