The Earls of Mar are a different clan and family from the Earls of March. Lots of stuff on Google/Wikipedia, etc.
Perhaps the most famous (or notorious?) Earl of Mar was "Bobbing John":
"John Erskine, 23rd Earl of Mar, lived from 1675 to May 1732. An important landowner and politician, he played an key role in the Act of Union between Scotland and England in 1707; then led the Jacobite cause in the 1715 uprising. His frequent changes of sides led to him being known as "Bobbing John", and to his losing the trust of just about everyone."