All I know about Mary Cassady is that she is mentioned in the will of William Carson.
William Carson will (1792), Fayette County, Pennsylvania Will Book 1:21, Fayette County, Pennsylvania. Names his eldest John John Carson, daughters Elizabeth Jane and Sarah, wife Sarah, son Alexander Carson. Names Mary Cassady who is to be supported and should she outlive William’s wife Sarah, Mary is to be buried at his cost. To John, five shillings as the remaining part of his full share of the estate. The same sums to daughters Elizabeth Jane and Sarah To son Alexander 50 acres of land joining land of Joseph Roabe and John McLala Executors, sons John and Alexander. Signed William (x) Carson his mark Date Signed 19 Jan 1791Date Recorded13 Jan 1792 Witnesses John McClellandHenry Robinson