She was the first of two Maritime Commission type S3-S2-BP1 ships built for the US Army Signal Corps near the end of World War II.

[5] After completion for the US Army Signal Corps in February 1946, Neptune was handed to the Maritime Commission and placed in the James River reserve fleet on 2 March 1946.

[6][7][8] In 1952 Neptune was assigned to Project Caesar, the unclassified name for the installation phase of SOSUS.

[6] The ship then went to the Bethlehem Steel Co. in Baltimore, Maryland for a number of modifications: e.g., electric cable machinery (in place of steam), precision navigation instrumentation, and a helicopter platform over the fantail.

The AT&T CS Long Lines was used on occasion to lay trunk cable at a daily cost of $30,000 vice Neptune's $19,200.

Scheduled Project Caesar work required a minimum of three Navy cable ships.

Inactivated in 1991, the same year in which the SOSUS mission was declassified, she was placed in the James River reserve fleet near Ft. Eustis, VA on 24 September 1991.