Tolkien didn't invent the fantasy genre so much as he nudged it toward the mainstream. There wasn't much of a market for fantasy literature (of the D&D variety) before LOTR was released, so now people argue that LOTR was the first significant work of the genre. Tolkien may have been responsible for creating the concept of a trilogy for fantasy/sci-fi/adventure, though you can argue that he really wanted LOTR split into 6 books, not 3.
But if Tolkien invented anything, it's Middle Earth, one of the most real alternative universes created in the fantasy genre. The creation of a language, the precise dating of events, the recording of noble lineage, etc., make it seem that Middle Earth does indeed exist, or did exist, in some time and place.