Tatlock was born in Stamford, Connecticut, in February 1876, the son of Florence (Perry) and The Rev.

The book for which he is chiefly remembered is his posthumously published study of Geoffrey of Monmouth, The Legendary History of Britain.

[4][5] Tatlock married Marjorie Fenton in 1911, and they remained together until her death in 1937.

The pair had two children: a son, Hugh (1912–2005), and a daughter, Jean (1914–1944).

Jean became a psychiatrist, writer, and a member of the American Communist Party, and she was known for her romantic relationship with Manhattan Project leader J. Robert Oppenheimer in the 1930s.