Except for a period out of service at Norfolk Navy Yard 6 May to 5 December 1912, Neptune operated along the east coast and in the Caribbean from Boston to Guantanamo Bay, Cuba, supporting ships and installations of the Atlantic Fleet into 1913.

Continuing her replenishment operations, she made two coaling voyages to Vera Cruz, Mexico in the spring and summer of 1913, returned to Norfolk 8 September, decommissioned there 13 October, and was placed in reserve.

Arriving at St. Nazaire on 8 June, the ship offloaded her cargo and disembarked her troops, the first American forces to land in Europe for service against the enemy.

Neptune made another coaling voyage to the west coast before departing Philadelphia on 27 August for an eleven-month cruise as part of the Train, Pacific Fleet.

She visited Pearl Harbor from 8 September to 25 October and cruised to Valparaíso, Chile in January and February 1921 to coal units of the Battle Force engaged in maneuvers off the Chilean coast.