Mare Island Light stood on the north shore of San Pablo Bay at the entrance to the Carquinez Strait, marking an important navigational approach where bay and strait met. Built in 1873 and designed by Paul J. Pelz, it was part of a group of similarly styled California lighthouse stations. Its function declined after the better-sited Carquinez Strait Light opened in 1909, and it remained in service until 1917 before being demolished in the 1930s. The site later gained military importance during World War II, when a Navy Direction Finding Station operated there from 1942 to 1945, reflecting a shift from coastal navigation aids to naval communications.