William's late mother, Princess Diana, and great-grandmother, the Queen Mother, also were Washington's cousins.
Middleton's other American relatives include explorer Meriwether Lewis, who teamed with William Clark for their renowned 19th-century Western expedition. He's a ninth cousin seven times removed.
The society also says Middleton is a 13th cousin three times removed of World War II General George S. Patton.
Prince William and Middleton are 15th cousins, through Sir Thomas Fairfax, who died during Henry VIII's reign.