I'm pretty positive that the name comes from the place our ancestors used to live. Mardon means "Mare's Hill" in old english (that's mare as in horse). There's a hill of that name in Devon, at the edge of Dartmoor National Park and near the village of Moretonhampstead. I've never been, but it seems like a nice place, and has a prehistoric stone circle and an iron-age standing stone.
Did you know there's a Mardon coat of arms? It has choughs on it and you can find it, together with a lineage going back to 1689 in Burke's Landed Gentry, a reference book and website.