I have not yet found a death date or place.
I found an online obituary for Martha's son, Charles, from 1920:
JOHNSTON, Charles W.
Charles W. Johnston died at his home in New Albany, April 18. He is survived by his wife and six children: Walker B. Allen, Mrs. George Weaver, of Ithaca, N. Y.; Luther, of Endicott, N. Y.; Arthur and William of Elmira, N. Y., and Olive at home, also an aged mother, Mrs. Martha Bensley, one sister, Mary Decker of Newark. N. J., and one brother, James Johnston of Wyalusing. He will be greatly missed by his family and many friends in New Albany and vicinity. (Daily Review, Towanda PA., Friday, April 30, 1920) Note- Son “William” should read Billy; his first name was not William.
This indicates Martha lived until at least April of 1920. Martha is found living with daughter Mary DECKER in the 1910 census. I have not found Martha in the 1920 census (yet).