There are at least 11 Keithley families that can be identified, living in Harford, Cecil, and Talbot counties. Two of these families, those of Francis Keithley (c. 1730), and a Keithley of unknown first name (c. 1650), have been documented somewhat. By the middle-to-late 18th century, family connections can be proved. It is likely that some of these families are related, and that there are really only about three separate families. All of these people are English.

(brothers?)|Francis (c.1680)-->William (c.1700)-->William (c.1750)
                 |Thomas (1687)--->William (1718)---->Thomas (1745)
                                                                                    |
                                                                                    |
                                                                                    |     |William (1769)
                                                                                    |-->|Elijah (1770)
                                                                                          |Abraham (1775) 
                                                 

                                                                       |William (1770)
                                                                       |John (1773)
                                                                       |Henry (1775)
Francis (c.1730)-->|John Thomas (c.1751)-->|Thomas (1776)
                                                                       |Richard (1778) ------
                                                                       |Jonathan (1780)          |
                                                                       |Francis (1782)            |
                                                                       |Sarah (1788)              |
                                                                                                          |
                                                                                                          |
                                               
                                                                                               |John (1803)
                                                                                               |Thomas (1805)
                                                                                               |William (1808)
                                                                          (Richard)--->|Richard (1812)
                                                                                               |Edward (1814)
                                                                                               |Elizabeth (1817)
                                                                                               |Jonathan (1821)

Humphrey (c.1660)
John (c.1710)
Sarah (c.1720)
Sarah (c.1725)
Mary (c.1730)
Henry (c.1744)         
         

Although these early Maryland families have not been well documented, the most knowledgeable resource for them is Frank Deis at Rutgers University.  See his pages at the following addresses:

Keithleys of Talbot County, MD

Keithleys of Harford and Cecil Counties, MD

Keithleys of Washington, D.C.

Keithleys in Congressional Cemetery

1880 Soundex, Washington, D.C.