I know this is an old post but saw it while researching my family in PG Co. I grew up in "Marlborough" which is really Upper Marlboro, PG CO, MD now. There are several old cemeteries in the town or adjacent. The three main ones I know of for the local families are:
St. Barnabas Church Cemetery (Queen Ann area in Upr Marlboro mid 1800s)
Mt Carmel Church
Trinity Episcopal on Rectory Lane in the town proper.
If you have any more questions about Upper Marlboro area feel free to ask. Plus I am moving back there after being away 20 yrs so if you need a "local" let me know.
Hope this helps.
Donna Hyatt (granddaughter to Louis Buck and Emma V. Chaney)