When William Theodore Berrett was born on 18 November 1899, in Iona, Bingham, Idaho, United States, his father, William Silas Berrett, was 21 and his mother, Rachel Thomas Beach, was 20. He married Harriette Smith Ballif on 20 February 1924, in Salt Lake City, Salt Lake, Utah, United States. They were the parents of at least 5 daughters. He lived in Menan, Jefferson, Idaho, United States in 1920 and Menan Election Precinct, Jefferson, Idaho, United States in 1940. He died on 22 April 1987, in Idaho Falls, Bonneville, Idaho, United States, at the age of 87, and was buried in Little Butte Cemetery, Annis, Jefferson, Idaho, United States.