Webster Treat was born 4 December 1827 in Frankfort, Maine, the son of Colonel Robert Treat and his wife Joanna G. (Crosby) Treat. His paternal grandparents were Joshua and Lydia (Buck) Treat. His maternal grandparents were John and Sarah (Wheeler) Crosby.
He married Hannah Jane Parker, who was born about 1829 in Maine, the daughter of Lemuel and Mary (Black) Parker. Webster and Hannah Jane Treat had 4 children:
- Helen, born 2 November 1852 in Maine; she married John Franklin Godfrey.
- Hayward C., born 5 Oct 1855 in Maine; he married Pauline Eugenie [Borie ?] of France
- Warren Franklin, born March 1862 in Maine
- Percival Webster, born 26 November 1865 in Maine; he married Sara, born in California