What happens if a mentally incompetent or mentally challenged adult, who is not in charge of their money (or has a legal guardian) wins a large sum of money?

A conservator handles the money, for the benefit of the individual. That is overseen by an agency or a court.

Thanks for answering, How, then, would the conservator or guardian-pro-tem or whatever, be appointed?

What would the limits be on the mentally incompetent adult’s access to the money, due to their inability to make decisions about money?

They vote him President?

@Yellowdog I believe they’re usually appointed by a court.

Its placed in a Trust account and handled by responsible Guardians etc
I know of a case where this lady in a group home received an inheritance.
Her inheritance was placed in a Trust acct and managed for her to have increment amounts to live on and spend comfortably without loss of the entire amount.

@Yellowdog “What would the limits be on the mentally incompetent adult’s access to the money, due to their inability to make decisions about money?”

In many cases where an appointee handles an adult’s finances the adult in question is often given a modest weekly or monthly allowance to spend at their discretion.