After trials and exercises in New England waters, L-1 was assigned to the Atlantic Submarine Flotilla operating along the East Coast.

When the United States entered World War I, L-1 underwent extensive overhaul at Philadelphia, Pennsylvania, to prepare her for vital tasks ahead.

Departing New London, Connecticut, on 27 November 1917, the submarine sailed for European waters to protect Allied shipping lanes from U-boat attacks.

Following brief operations in the Azores, L-1 sailed to the British Isles for patrol duty out of Bantry Bay, Ireland, beginning in early February 1918.

With the defeat of the Central Powers, L-1 departed the Isle of Portland, England, on 3 January 1919 and arrived Philadelphia on 1 February.