From what I can make out from this source
http://www.meertens.knaw.nl/nfd/detail_naam.php?naam=MondriaanThe name began as a French surname, Monreal. This became Mondriaal, Mondria and Mondriaan.
Maybe someone who can read Dutch will confirm this (or not).
I'm assuming that Monreal is a version of the French place name/surname Montréal.