William Tennant Mortlock (1858 – 17 August 1913) was a South Australian grazier and politician. Mortlock was born near Port Lincoln, the son of William Ranson Mortlock. He was educated at St Peter's College, Adelaide and Jesus College, Cambridge. He read for the law at the Inner Temple while in England, but returned to South Australia in 1883 and did not pursue his legal studies further. He worked on his father's Yudnapinna Station, near Port Augusta, and he increased the family's pastoral property after inheriting it upon his father's death in 1884. In 1891 he purchased Martindale Hall at Mintaro, which would become his family's main station.