I'm no expert, but on this web site: http://catorfamily.com/genealogy/earlycators.html George Cator is said to have lived in Milingham, PGC, Maryland in 1840. In my 1878 atlas, George Cator is listed on page 22 in the Spaldings District (& in my 1861 map he's in the same location, though it appears on page 14). It looks like he lived between the Suitland and Silver Hill post offices (Silver Hill no longer has a post office). It looks like he lived off Silver Hill Rd. west of Suitland Rd. This area would probably be called Suitland today.

There is nothing about Milingham on this map or an 1861 map I have. It's possible that it was a small neighborhood with no post office, so it wasn't named on the map. It does appear to be fairly well populated for tose times and it's not far from the DC border.