On a visit to Bernardsville, Hammond met Mary Picton Stevens (1885–1915). They were married in Hoboken on April 8, 1907, their marriage lasting up until Mary's death in the sinking of the Lusitania. Mary was the daughter of John Stevens (1856–1895), oldest son of Stevens Institute of Technology founder Edwin Augustus Stevens and grandson of inventor John Stevens, and Mary Marshall McGuire (1850–1905). The Hammonds settled in a forty-seven-room mansion in Bernardsville in 1908. Hammond and his first wife had three children:
