My HC for Mandalore (before the Clone Wars period, when they actually had a Manda'lore and not Duchess Whitewashed-Pacifism) is that it's a sort of elective monarchy like Celtic Ireland (or any Celtic polity during the Classical Period) or like the Grand Duchy of Poland-Lithuania, which means it's not hereditary, but based on the personal characteristics and charisma of the incumbent. When the previous leader dies or is dismissed, an assembly votes the new one.
The HC also includes the fact that like in Celtic kingship, the Manda'lore is an absolute leader only in war, while in peace each clan is mostly independent and self-governs.
This also means a lot of infighting, so it's a mixed bag.