Mary was John Cox's second wife. They married in Henrico Co. VA (I'll have to double-check where in that county, though). "John Coxe, Sen. marryage with Mary Kennon" is listed under "Accot of Lycenses returned to Town this 25th of 7ber, 1682"
I don't know if she was born a Kennon or had been married before.
John Cox was the son of William Cox and Elizabeth ____. Elizabeth later married William Elam and was mentioned in William Elam's will as his son-in-law, which at that time usually meant step-son.