A mentalist is a general term for a type of performer who uses real or pretended skills to demonstrate some kind of mental ability beyond the realm of what's usually considered possible. So, a mentalist might demonstrate an extraordinary memory, the ability to move objects with their mind, a heightened sense of empathy or intution, the ability to see the future, and so on.
A hypnotist has a more specific definition in that it is someone who uses hypnosis. That could be in a performance context or in therapy. Of course, these terms aren't mutually exclusive, a mentalist might also be a hypnotist.