Oxford* has:
MONDON: > MUNDON, from the place-name M. in Essex
Oxford** once more: MUNDON: "protection hill", i.e. probably "raised ground safe from flooding, or protected by fencing"; Old English "mund" + "dun"
* P.H.Reaney, A Dictionary of Surnames, Oxford 1997
** A.D.Mills, Dictionary of Place-names, Oxford 1998
Andy ;—)