I will answer this question as a general matter in the context of Utah law because I practice divorce and family law (and thus I deal with child custody disputes daily), so bear in mind that my answer to this question does not (and cannot) apply in all jurisdictions or in specific situations. If you are in a situation where you find yourself 100% mentally disabled and want to seek full custody of your children, consult independent counsel for your specific situation.
I am not certain what you mean by 100% mentally disabled, but I would define it as being completely and totally disabled. And if a parent is completely and totally disabled, whether mentally or physically, it is hard to imagine a reasonable scenario in which that completely and totally disabled parent could be considered fit to exercise any degree of legal or physical custody of his/her children. The way Utah law analyzes the question is covered by Utah Code §30–3–10(6):
(6)
(a) Except as provided in Subsection (6)(b), a court may not discriminate against a parent due to a disability, as defined in [Utah Code] Section 57-21-2, in awarding custody or determining whether a substantial change has occurred for the purpose of modifying an award of custody.
(b) The court may not consider the disability of a parent as a factor in awarding custody or modifying an award of custody based on a determination of a substantial change in circumstances, unless the court makes specific findings that:
(i) the disability significantly or substantially inhibits the parent’s ability to provide for the physical and emotional needs of the child at issue; and
(ii) the parent with a disability lacks sufficient human, monetary, or other resources available to supplement the parent’s ability to provide for the physical and emotional needs of the child at issue.
With the factors of §30–3–10(6) in mind, it appears to me that 100% mental disability would significantly and substantially inhibits (if not fully negate) the parent’s ability to provide for the physical and emotional needs of the children. It is hard to imagine any amount of “sufficient” human, monetary, or other resources could compensate for a 100% mental disability to the point that a court could find that those resources sufficiently supplement the parent’s ability to provide for the physical and emotional needs of the child at issue. Otherwise stated, if a parent is 100% disabled, then then only way to compensate for that would be to entrust custody of the child to one or more fully abled people instead of the 100% disabled parent, which would thus disqualify the 100% disabled parent from exercising custody to any degree.
Utah Family Law, LC | divorceutah.com | 801-466-9277
https://www.quora.com/How-can-a-100-mentally-disabled-person-keep-full-custody-of-their-kids