The Neptune was the 'workhorse' of Earth's Starfleet during the mid 22nd century. The ships were designed for general purpose defensive duties. They were originally fitted with spatial torpeodes and plasma cannon. With a top cruise speed of warp 1.5 they had an average travel time between systems of up to several years. The Neptunes would rarely undertake such missions - they would typically use their interstellar capability to transit to a given system and then operate there for a considerable period, conducting local patrols at impulse or low warp. At the end of their mission the ship would then return home at warp speed. Such missions could last as much as ten years, and with a thirty year hull life a typical Neptune class vessel might only travel to two or three different systems throughout its life.
Like all other Human designed ships, the Neptunes were greatly outpaced by the NX class starships which began to appear in the 2150s. Even so they continued to operate through this 2150s and beyond. The ships were refitted throughout their lives to keep them up to date; they carried phase cannon, and some recieved photonic torpedoes to supplement or even replace their spatial torps.

Neptune classes served as part of Earth's defence force after the Xindi attack of 2153; at least one of these ships responded to the tresspass of a Klingon Bird of Prey in the system when the NX-01 returned to Earth in the aftermath of the attack.