Plenary Guardian Law & Legal Definition. Plenary guardianship is a guardianship in which the court gives the guardian the power to exercise all legal rights and duties on behalf of a ward, after the court makes a finding of incapacity.t states that “The court may appoint a plenary guardian of the person only upon a finding that the person is totally incapacitated and in need of plenary guardianship services.”. Additional Plenary Guardian Resources. View Guardianship Forms-Download Guardianship Forms in Minutes.