If your grandmother describes you as mannered, she means you are very formal, observing all society’s mandates. She might mean this positively, approving of your actions, or negatively, thinking you too formal.
Manners are the way we behave in society, how we speak and act with other people. Use mannered to say that someone has manners of a certain kind. You might be well-mannered, helping old ladies cross the street, while your brother is ill-mannered, throwing snowballs at passing cars. Mild-mannered means that your manners are gentle — kind and polite.
