There are several games with the Lovecraftian setting. Arkham Horror is long, often ridiculously so. There is a newer game out called Eldritch Horror which is still long, but far more feasible to fit into an evening than Arkham Horror. I have played Eldritch Horror, and it was quite fun. You're moving around a map of the world collecting clues and equipment to seal rifts/portals and keep the monsters from coming through to the world.
Another very popular Lovecraft-themed game is Elder Sign. Elder Sign can be played solo (1-8 players) and is much faster and more simple than the previous two mentioned. It's more of a dice game, I guess, where you have to roll certain sets and combinations of symbols for the different "locations" that give your different items and help score points. I played a game with seven people, and my experience wasn't good, but I won't say the game is bad because of that. With seven people chatting, the turns were taking a rather long time to go around. I've heard it's a lot better with 5 or less, but it really depends on your group. Here's a video:
I've not played Mansions of Madness, but I do own a game that a lot of my friends compare it to, Betrayal at House on the Hill. Mansions of Madness has a fixed board/map for the different scenarios. One player plays the monsters while the others are explorers searching for scenario-specific clues. It comes with nice miniatures for most of the monsters, I'm pretty sure, whereas Betrayal only has miniatures for the players and uses tokens for everything else.
It may not be based on Lovecraft's work, but Betrayal at House on the Hill is one of my favorite games. All players start out working together to explore a haunted house and raise their skills. As players search, they get to place tiles out to expand the floors of the house, so the board is always different. Eventually, an event called "The Haunt" occurs. The rules for the 2nd Edition contain 50 different Haunt scenarios, determined by what room the haunt was triggered from and which "Omen" card triggered it. Following The Haunt, one player becomes the Traitor and has his/her own scenario book that they have to go to another room and become familiar with their new objective. This is a "horror" game, so the Haunt involves the arrival or awakening of some sort of monster and/or the possession of one of the heroes (to become the traitor). The rest of the players have their own objective to win/survive and work together against the Traitor. It's quite fun.