You could make a case that the first main character who was a never-married, independent career woman was Our Miss Brooks, though again that independence and career were presented within an early 1950s social context.

But I agree with the main point about the Mary Richards character's subtle influence.  I was surprised to hear Samantha Bee say that MTM's portrayal of a single, female news producer let the girl Bee used to be know that she could grow up to do interesting things.